What is an API? A plain-English guide

An API is the way one piece of software talks to another. It is how your app takes payments, shows a map, signs you in with Google, or calls an AI model, all without rebuilding those things yourself. This guide explains what an API is, how it works, the main types, and why nearly every modern product depends on them.

By Kanika Mathur, Head of Service Delivery

Reviewed by Resourcifi engineeringPublished Feb 16, 2026Updated Feb 16, 20268 min read

Engineering

Key takeaways

The short version

An API (application programming interface) is a defined way for two pieces of software to talk to each other, a contract that says what you can ask for and what you get back.
It works as a request and response: your software asks an endpoint for something, the other system does the work, and sends back data, usually as JSON.
You use APIs to reuse what already exists, payments, maps, sign-in, messaging, and AI models, instead of building each from scratch.
The main styles are REST (the most common), GraphQL, the older SOAP, and webhooks for event notifications. REST over HTTP covers most needs.
APIs are the backbone of modern software: they connect your app's frontend and backend, link you to third-party services, and let other developers build on your product.

What an API is

An API, or application programming interface, is a defined set of rules that lets one piece of software request services or data from another. Think of it as a contract: it specifies what you can ask for, how to ask, and what you will get back, while hiding all the complexity behind it. A useful analogy is a restaurant menu and waiter. You order from the menu (the API), the kitchen (the other system) does the work, and the waiter brings back your dish (the response). You never need to know how the kitchen runs.

How an API works

Most modern APIs work over the web as a request and response. Your software sends a request to a specific URL called an endpoint, often using an HTTP method such as GET to read data or POST to send it, along with any parameters and an authentication key. The receiving system processes the request and sends back a response, typically structured data in JSON, plus a status code that says whether it worked. Your app then uses that data, for example to show a map, a price, or an AI-generated answer.

Endpoint: the URL you call for a specific resource or action.
Request: the method, parameters, and authentication you send.
Response: the data you get back, usually JSON, with a status code.

Types of API

APIs come in a few common styles, and most apps use REST. REST organizes everything around resources accessed over standard HTTP methods and is simple and widely supported. GraphQL lets the client ask for exactly the data it needs in one request, which reduces over-fetching. SOAP is an older, stricter, XML-based protocol still used in some enterprise and financial systems. Webhooks flip the model: instead of you asking, the other system notifies you when an event happens. APIs are also classed by access, public, partner, or private.

Common API styles
Style	What it is	When to use it
REST	Resources over standard HTTP, usually JSON	Most apps and public APIs
GraphQL	Client asks for exactly the data it needs	Complex or data-heavy clients
SOAP	Older, strict, XML-based protocol	Legacy enterprise and finance systems
Webhooks	The server notifies you on an event	Real-time updates and integrations

Why APIs matter

APIs matter because they let software reuse capabilities instead of rebuilding them, which is how modern products get built quickly. They connect your app's own frontend and backend, integrate third-party services like payments, maps, sign-in, and AI models, and let outside developers build on top of your product, turning it into a platform. Almost every app you use is really a set of APIs working together behind the scenes, which is why API design and security are core engineering concerns, not afterthoughts.

The industry has taken note. Postman's 2025 State of the API Report⁴ found that 83% of development teams are now building API-first, and one in four developers is already designing APIs with AI agents in mind. That shift matters for anyone building a product today: your API design choices shape what your software can do tomorrow, not just this sprint.

APIs are central to how we build; see our custom software and AI application development work, where well-designed APIs connect everything.

Frequently asked

API questions

What is an API in simple terms?

An API, or application programming interface, is a defined way for two pieces of software to talk to each other. It is like a contract that says what you can ask for and what you will get back, while hiding the complexity behind it. A common analogy is a restaurant: you order from a menu (the API), the kitchen does the work, and the waiter brings back your dish (the response), without you needing to know how the kitchen runs. APIs are how apps reuse other software's capabilities.

How does an API work?

Most modern APIs work over the web as a request and response. Your software sends a request to a specific URL called an endpoint, usually with an HTTP method such as GET to read data or POST to send it, along with parameters and an authentication key. The other system processes the request and returns a response, typically structured data in JSON plus a status code indicating success or failure. Your app then uses that data, for example to display a map, a price, or an AI-generated answer.

What is a REST API?

A REST API is the most common style of web API. REST, representational state transfer, organizes everything around resources, such as users or orders, that you access using standard HTTP methods like GET, POST, PUT, and DELETE, usually exchanging data as JSON. It is popular because it is simple, predictable, and supported by virtually every language and tool. When people talk about calling an API today, they usually mean a REST API over HTTPS.

What is the difference between REST and GraphQL?

REST exposes fixed endpoints that each return a set shape of data, so getting everything you need can take several requests and may return more than you use. GraphQL exposes a single endpoint where the client asks for exactly the fields it wants in one request, which reduces over-fetching and under-fetching. REST is simpler and ubiquitous and fits most apps; GraphQL shines for complex, data-heavy clients that need flexible, precise queries. Many systems use REST, and reach for GraphQL where its flexibility pays off.

What is an API endpoint?

An API endpoint is a specific URL that represents a resource or action you can call. For example, one endpoint might return a list of products and another might create an order. You send a request to the endpoint with an HTTP method and any parameters, and it returns a response. A single API is usually made up of many endpoints, each handling a particular piece of data or operation, documented so developers know what each one does.

Why are APIs important?

APIs are important because they let software reuse capabilities instead of rebuilding them, which is how modern products are built quickly and reliably. They connect an app's own frontend and backend, integrate third-party services such as payments, maps, sign-in, and AI models, and let other developers build on top of your product so it becomes a platform. Almost every app is really a set of APIs working together, which is why good API design and security are core engineering concerns.

Kanika Mathur

Head of Service Delivery, Resourcifi

I am Kanika Mathur, Head of Service Delivery at Resourcifi. Almost every system we build is held together by APIs, between frontend and backend, to third-party services, and increasingly to AI models. This guide is the plain-English explanation I give non-technical stakeholders so they understand what their product depends on, grounded in how our engineers design APIs on real builds since 2017.

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Sources

MDN Web Docs, API glossary and web API guides.
IBM, What is an API?
Postman, What is an API? (request and response, API types).
Postman, 2025 State of the API Report (5,700+ respondents; 83% API-first adoption; 1 in 4 developers designing for AI agents).