What is an API? (Application Programming Interface)

I’ve probably used an API dozens of times today, without even realizing it. When I use my phone to order a burrito, check the weather, or see how the Ravens are doing, that e-magic happens because one program can talk to another through an API (application programming interface).  

APIs are everywhere in the digital world, quietly powering the tools and services we use every day. But what exactly are they, and how do they work? In this guide, I’ll break down everything you need to know about APIs—and you won’t need a coding bootcamp under your belt to follow along.

Table of contents:

What is an API?

An API (application programming interface) is a set of rules laying out how one piece of software can communicate with another. 

Imagine two applications cut off from each other by a chasm: an API is like a bridge letting data, features, and services flow from one side to the other. 

Because apps are complicated little darlings, APIs have to be designed with specific rules that define where to connect (entry points), how to communicate (protocols), and what language to speak (data formats).

Here are some key API terms you’ll be seeing a lot of moving forward.

How do APIs work?

APIs act as messengers between two software systems. When you check your weather app for the current temperature and forecast, it’s pinging an API to request data from a dedicated weather service. That data gets sent back to your app, and you find out whether you need an umbrella, a sweater, or to just never leave the house again. 

APIs let different software components built by different developers in different environments work together without needing to understand the others’ internal systems. 

Without APIs, each piece of software would be locked in its own little chamber. Apps can be written in various coding languages, use their own data-encoding formats, and operate within unique software architectures. So, even if two programs wanted to share information, they’d have no standardized way to ask for it or understand the response. APIs provide that standardized interface.

Common protocols for building APIs

A good way to make sense of APIs is to break them down, LEGO-style, into their building blocks: protocols. Basically, a protocol determines the rules and standards for how an API exchanges data.

Without sinking us too deeply in the technical mud, let’s look at the most widely used protocols used to design APIs. 

• SOAP (simple object access protocol): SOAP is a protocol for exchanging structured information, such as financial transactions or health records, across web services. It’s platform-agnostic and built to handle highly structured, secure data transfers.

• REST (representational state transfer): REST is an architectural style that uses standard HTTP methods (like GET, POST, PUT, DELETE) to interact with resources represented as URLs (endpoints). Unlike SOAP, REST is lightweight, flexible, and focuses on simplicity and scalability. It typically uses JSON (JavaScript Object Notation) for data exchange, though XML and other formats are also supported.

• RPC (remote procedure call): RPC is a protocol that lets one program call a function or service in another program as if it were local code, without needing to understand the technical details of how that remote program works. If an app uses a chat feature that’s really hosted by another service, clicking “send message” might trigger an RPC that executes the sending function on that remote server

• XML-RPC: XML-RPC is the same thing as a remote procedure call protocol, but it’s only encoded in XML (extensible markup language), which is a common format used to make documents that can be read by a human or a machine.

• JSON-RPC: JSON-RPC is similar to XML-RPC but uses JSON instead of XML for data formatting. Like XML, JSON is meant to be easy to parse for humans and machines alike.

• WebSocket: WebSocket is a protocol that enables real-time, two-way communication between clients and servers. Unlike traditional HTTP requests where the client asks and the server responds once, WebSocket keeps an open connection so both sides can send messages back and forth.

• GraphQL: This query language for APIs allows clients to request only the data they need. With GraphQL, you can grab just the name and email of a particular app user, for example. Because no extra data gets sent, you’ll save bandwidth and boost performance, which is especially important for mobile apps or slow networks.

What’s a real-life example of an API?

I’ve probably used about seven APIs in the past 30 minutes, but they’re so seamlessly integrated into my life, I’d never realize it. To really grasp the concept, it helps to think of a real problem you might use an API to solve. 

Say you want to cut your workload by automatically sending a follow-up email whenever someone fills out a Google Form. Here’s how an API might pull the responses on a form into an email platform like Outlook:

1. A user submits a response to your Google Form.

2. The form sends the data to your email platform (Outlook, in this case) via an API.

3. The email platform receives the data and triggers an automated email notification.

Even a small-scale API like this will save you time and effort, but it can be quite tricky to build from scratch. That’s where business automation tools like Zapier come in. With Zapier, you can automate workflows like this one without writing a single line of code. And that’s just a simple example: Zapier connects to thousands of APIs, so you can set up your automations in a visual workflow builder, skip over about a dozen IT bottlenecks, and connect your apps. Put simply: Zapier is like the API for APIs.

Here’s a template for the exact workflow automation I just described, along with a couple of souped-up variations.

Again, we’re really just scratching the surface of API value, so here are some more examples of API use cases, broken down by the departments that would probably implement them.

Types of APIs

There’s no one-size-fits-all approach to building an API, and two devs might go about coding one in entirely different ways. This isn’t the Wild West, though, and there are some standard API types that most examples fall under.

• Composite APIs: Composite APIs combine multiple endpoints or services behind a single call, allowing clients to get data from several sources at once while masking the technical complexity.

• Open or public APIs: Public APIs are available to external developers and the public, and are often used to encourage innovation or third-party integrations (e.g., Google Maps API).

• Internal or private APIs: Unlike public APIs, private APIs are used within an organization to connect internal systems or services, improving efficiency without making endpoints available to the public (which might expose sensitive information). It’s the same thing as a public API, but operating only for internal teams.

• Partner APIs: Partner APIs can be thought of as a blend of public and private APIs. They’re shared selectively with external partners and only with agreements in place. 

• Data APIs: A data API works like APIs more generally, but its only purpose is to provide access to datasets, meaning applications can query, retrieve, and update information. Since it does this without directly accessing the database, there’s less of a security threat. This kind of API has become increasingly important with the emergence of generative AI.

• Database APIs: Similar to data APIs, database APIs let you interact with a database like NoSQL, allowing applications to read, write, and manage stored data efficiently.

• Remote APIs: A remote API enables systems to communicate over a network, and it’s common for applications to connect to cloud-based resources or services via remote APIs.

• Local APIs: Local APIs only operate inside a single device or local environment. They’re more private and are popular with smaller teams working with proprietary information.

APIs vs. related concepts

APIs are really just one of a number of app-bridging technologies that have sprung up ever since computers started computing. If you can tell an API from an SDK, then you’ll be better-equipped to think about which approach makes the most sense for your projects. So, let’s look at how APIs differ from other integration methods.

APIs vs. web services

Web services are actually a type of API in which communication is restricted to a network (e.g., the internet). They rely on protocols such as HTTP and SOAP to enable interactions between systems, and they typically (but not exclusively) format data with XML.  

APIs vs. SDKs

An API defines how two software components interact, while an SDK (software development kit) provides tools and libraries for developers to build applications. APIs focus on communication between apps, whereas SDKs provide the components that engineers use to build software.

APIs vs. webhooks

APIs allow back-and-forth communication between applications, while webhooks are an “event-driven” type of API. So, instead of waiting for a request like APIs do, webhooks send data automatically when a specific trigger occurs (like when someone clicks a CTA on a website). 

APIs vs. microservices

Microservices are small, independent services that are stitched together into a broader application. APIs, on the other hand, define how these services communicate. While microservices solve specific scaling and resource efficiency problems, APIs set up interactions between them.

Best practices for building APIs

Creating an API means defining endpoints, methods, and authentication protocols, plus writing documentation that tells people how to use it. You’ll also have to test the API to make sure it doesn’t crash your server the first time two requests come in simultaneously. (Not that I’d know.)

This isn’t the place for a full tutorial on API design—people get degrees to figure that stuff out. But if you do have the dev chops to build your own API, here are some best practices to keep top of mind.

Define clear endpoints and methods 

An API endpoint is the URL where your API lives. Methods such as GET, POST, PUT, and DELETE define the actions that can be performed on the API. A GET request gets data, while a POST request usually posts (creates) a new record, and so on. When designing endpoints, aim to make them easy for developers to use by following a consistent naming convention.

Build in robust authentication and authorization

Because APIs offer access to your platform, data, and other assets, security is important—and authentication mechanisms like API keys, OAuth tokens, and JWT (JSON Web Tokens) are common ways of making sure your API is only accessed by approved users.

Test rigorously

Testing is key to ensuring your API performs reliably under different conditions. Usually, you’d start with unit tests that verify individual components, move on to integration tests (to confirm that all parts work together), and then test different edge cases (e.g., what happens if someone sends bad data or exceeds your base rate limits). Automated testing frameworks can save time and improve accuracy.

Plan for scalability and versioning

If you’re building something beautiful, useful, or cool, your user base will grow, and your API needs to handle increased traffic without your performance slowing to a crawl. Design your API with scalability in mind by using caching strategies, load balancing, and efficient database queries. Additionally, plan for versioning from the start, by following the common strategy of labeling your API as v1 and incrementing the version number as needed. This allows you to introduce updates or changes without breaking existing integrations.

Even if you don’t personally plan on making an API, knowing at least this much will help you speak to the devs you hire to build one for you. For a deeper dive, check out Zapier’s comprehensive introduction to APIs.

How to create API documentation

I’ve always been in the “throw the manual away and start fiddling with it” camp when it comes to playing with tech. But for APIs, I’ll be the first to admit that good documentation (basically a manual for API use) is an absolute must-have.

By writing documentation with clear instructions on endpoints, methods, parameters, and examples of requests and responses, developers will be able to quickly get up to speed on what your API offers and how it can contribute to their projects. 

Documentation does include technical details like code samples, but you’ll also want to give readers, in plain language, a practical guide on what your API does, plus real-world use cases and tips for troubleshooting.

In my experience, strong documentation comes from spending time playing around with your own API as if you were an end user. More specifically:

• See if you understand each endpoint and what it’s meant to accomplish.

• Try to build a simple toy project that uses a few of the endpoints.

• See if you can break the API (e.g., What happens if you pass in numbers as strings? Do the error messages make sense?).

• Test the API under different conditions. How much does it slow down as you increase the volume of data you’re sending? Can you get it to return data it’s not supposed to?

It’s also worth pointing out that your documentation needs to be up-to-date and accurate. So if you’re constantly pushing tweaks and fixes, make that clear whenever you publish new versions.

And if you’d like to know more, check out Zapier’s full guide on how to write API documentation (the right way).

Skip time-consuming API work with Zapier

APIs are the links that turn a series of isolated applications and services into an ecosystem, but building and maintaining them can be a Herculean endeavor at scale. 

If you’re looking to move data and leverage services without reinventing the wheel (or putting your devs through an integration gauntlet), Zapier offers a no-code solution for connecting apps and automating workflows. With Zapier, you can integrate over 8,000 apps and AI tools, giving you all the benefits of APIs without needing to know how they work on the backend.

If you want to know more, learn how you can use Zapier to connect and orchestrate your tech stack into automated, end-to-end business processes.

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