Last updated: May 2026
An API (Application Programming Interface) is a set of rules and definitions that lets one software system request data or services from another. APIs are the connective tissue of modern software — every time a mobile app shows live data, a website logs you in with Google, or a SaaS tool talks to your CRM, an API is doing the work in the background.
This guide covers what APIs are, how they work, the main types you'll encounter, real-world examples, security fundamentals, and how the rise of AI is reshaping how APIs are built and consumed. Whether you're a developer evaluating tools or a product leader trying to understand the integration landscape, you'll leave with a clear mental model of the API ecosystem.
An API (Application Programming Interface) is a defined set of rules that allows two software components to talk to each other. It specifies what requests can be made, how to make them, what data they need, and what data they return — without requiring either side to know how the other is built internally.
The most common analogy is a restaurant. You sit at a table with a menu of options. The waiter is the API: you tell them what you want, they take that request to the kitchen, and they bring back what you ordered. You don't need to know how the kitchen works — only what's on the menu and how to order from it.
In software terms:
This abstraction is what makes APIs so powerful. The kitchen can change its recipes, hire new chefs, or move buildings entirely — and as long as the menu stays the same, you keep getting the meal you ordered.
Most modern APIs work through a request-response cycle over HTTPS. Here's what happens when an app fetches your latest weather forecast:
GET https://api.weather.com/v1/forecast?city=London.A typical JSON response for that weather call might look like this:
{
"city": "London",
"current": { "temperature_c": 14, "conditions": "Cloudy" },
"forecast": [
{ "day": "Mon", "high_c": 16, "low_c": 9 },
{ "day": "Tue", "high_c": 17, "low_c": 10 }
]
}The same conceptual flow underpins almost every API interaction you encounter — whether it's authenticating with Google, charging a credit card with Stripe, or pulling customer records from your CRM.
APIs solve one of software's hardest problems: letting independent systems work together without exposing their internals. That sounds abstract, but the business consequences are very concrete.
Put another way: APIs are why a developer can build an app that books flights, charges a card, sends a confirmation email, and uploads a receipt to your accounting tool — all in a weekend. Each of those capabilities is somebody else's API.
APIs are most usefully categorised two ways: by who can use them (access) and by how they work (protocol or architecture).
This is where most of the technical decisions live. The five protocols you'll encounter most often:
|
Type |
Best for |
Data format |
Strengths |
Trade-offs |
|---|---|---|---|---|
|
REST |
Most public web APIs |
JSON (usually) |
Simple, cacheable, ubiquitous tooling |
Over- or under-fetching data |
|
SOAP |
Enterprise / regulated systems |
XML |
Strict contracts, built-in security |
Verbose, heavier, older tooling |
|
GraphQL |
Mobile apps, complex frontends |
JSON |
Client picks exactly the fields it needs |
More complex caching and ops |
|
gRPC |
Internal microservices |
Protobuf (binary) |
Very fast, strongly typed, streaming |
Browsers can't call it directly |
|
WebSocket |
Real-time chat, live dashboards |
JSON or binary |
Persistent two-way connection |
More server state to manage |
For a deeper comparison see SOAP vs REST: what's the difference, REST vs GraphQL, and when to use REST vs SOAP with examples.
APIs are easier to grasp through concrete examples. Here are five you've almost certainly used today, often without realising it:
For more, see 6 examples of APIs we use in our everyday lives.
Good APIs follow conventions that make them predictable for the developers who consume them. The most widely adopted principles include:
/customers/123/orders, not /getOrdersForCustomer?id=123. See best practices for naming REST API endpoints.Because APIs expose data and functionality directly, they are also the most common attack surface in modern applications. A solid API security posture combines several layers:
API documentation describes everything a developer needs to integrate with your API: endpoints, request and response shapes, authentication, error codes, and runnable examples. The current standard for documentation is the OpenAPI Specification (formerly Swagger), which produces a machine-readable schema that tools like Swagger UI, Redoc, and Postman can render as interactive docs.
Good documentation usually includes:
If you're evaluating documentation tools, see the 5 best API documentation tools.
Once a company has more than a handful of APIs, it usually needs API management — the operational layer that handles authentication, rate limiting, observability, versioning, and developer onboarding across an entire API portfolio.
The piece of infrastructure that does most of this work is the API gateway. A gateway sits in front of your APIs and handles cross-cutting concerns so each individual service doesn't have to:
The biggest shift happening to APIs right now is who's calling them. For the last twenty years, the primary API consumer was a human-built application — a mobile app, a SaaS dashboard, a backend job. Today, an increasing share of API traffic is generated by AI agents: large language models acting on behalf of users, calling tools, fetching data, and chaining actions.
This is creating a new layer in the API stack. The Model Context Protocol (MCP), introduced by Anthropic and now widely adopted, is the emerging standard for how AI models discover and call APIs at runtime. An MCP server exposes tools that an LLM can read and invoke; an MCP client (like Claude or ChatGPT) consumes those tools.
The practical implication: APIs that were designed for human developers now need to be discoverable, semantically clear, and safe enough to be called by autonomous agents. That changes a few things:
DreamFactory is an API generation and management platform that automatically creates secure REST APIs from any database, file system, or service — without writing code. Once those APIs exist, DreamFactory provides the API management layer around them: authentication, role-based access, rate limiting, logging, and a developer portal with auto-generated documentation.
For teams building AI-driven products, DreamFactory's MCP integration turns any of those generated APIs into tools an LLM agent can safely call — with the same governance controls that protect human-driven traffic.
If you'd like to see what an auto-generated REST API looks like for your own database, request a demo or browse the DreamFactory platform overview.
An API (Application Programming Interface) is a contract that lets one piece of software ask another piece of software to do something or share data. The classic analogy is a restaurant: you (the client) read a menu (the API specification), tell the waiter (the API) what you want, and the waiter brings it back from the kitchen (the server).
API stands for Application Programming Interface. "Application" refers to any software with a defined function. "Programming Interface" means the rules and tools that let other programs talk to that application without needing to know how it works internally.
When you check the weather on your phone, the weather app calls a weather service API to fetch live data. When you log into a website with Google, the site uses Google's OAuth API to verify your identity. When a Stripe checkout button accepts a card, it calls Stripe's payments API.
APIs are typically grouped two ways. By access: public (open to anyone), private (internal use), and partner (shared with specific companies). By protocol: REST, SOAP, GraphQL, gRPC, and WebSocket are the most common — each with different trade-offs around speed, structure, and use case.
An API is the general concept — any interface that lets software talk to other software. A REST API is one specific style of API that uses HTTP methods (GET, POST, PUT, DELETE) and follows REST architectural principles. REST is the most common API style on the web today, but it isn't the only option.
An API works through a request-response cycle. A client sends a request to the API endpoint, including any parameters, headers, and authentication. The server processes the request, fetches or modifies data, and returns a response — usually JSON or XML — along with a status code that indicates success or failure.
APIs let software systems communicate without sharing source code. That makes it possible to integrate third-party services, build mobile and web apps on top of existing data, automate workflows across tools, and ship new products faster. Modern software is built almost entirely on APIs.
API security covers the practices and tools that protect APIs from unauthorised access and abuse. Core controls include authentication (API keys, OAuth, JWT), authorisation (role-based access, scopes), encryption (HTTPS/TLS), rate limiting, IP allow-listing, and input validation to prevent injection attacks.
API documentation describes how to use an API: the available endpoints, request and response formats, authentication, error codes, and example calls. Good documentation usually follows the OpenAPI Specification (formerly Swagger) and includes interactive examples developers can try directly in the browser.
An API defines what you can do. An SDK (Software Development Kit) gives you the tools to do it. An SDK typically wraps an API in a language-specific library — so instead of constructing HTTP requests yourself, you call methods like stripe.charges.create(). SDKs make APIs easier to use; the API is still the underlying interface.