What Is an API? A Simple Explanation With Examples

Every time you check the weather on your phone, pay for something online or log in to a website with your Google account, an API is doing work in the background. You never see it. But without APIs, almost none of the apps you use every day would work the way they do.

If you have heard developers, vendors or your IT provider say “it connects through the API” and nodded along, this guide is for you. It explains what an API is in plain English, walks through a restaurant analogy that makes it click, shows real examples and covers the handful of terms you need to have a smart conversation about integrations.

What is an API in plain English

API stands for application programming interface. It is a defined way for one piece of software to ask another piece of software for data or to do something, and get a predictable answer back.

The key idea is that the 2 systems do not need to know how each other works inside. They only need to agree on the rules: what you can ask for, how to ask, and what the answer will look like. Those rules are the API.

The restaurant analogy

Picture yourself at a restaurant.

• You are the app that wants something.
• The kitchen is the other system, such as a weather service or payment processor, that has what you want.
• The menu is the API documentation. It lists what you can order and any options.
• The waiter is the API. You give your order to the waiter in a format the kitchen understands. The waiter brings back your food, or tells you the kitchen is out of salmon.

You never walk into the kitchen. You do not need to know the recipe, the oven temperature or who is cooking. And the kitchen can reorganize itself completely without changing the menu. That separation is exactly why APIs are so useful in software: each side can change its internals as long as the agreed interface stays the same.

Here is that same flow in software terms:

Your app --request--> API ---> Other system (server) Your app <--response-- API <--- Other system (server)

The request is the order. The response is the meal, usually delivered as structured data in a format called JSON.

Real-world API examples

You use APIs dozens of times a day:

• Weather apps. Your phone’s weather app asks a weather service’s API for the forecast at your location and displays the answer.
• Online payments. When you check out on a small business website, the site sends your payment details to a payment provider’s API, which replies “approved” or “declined.” Our guide to accepting payments on your website covers this.
• Maps. A store locator on a website embeds a map by calling a mapping service’s API.
• “Sign in with Google” or Apple. The website uses an API, usually built on the OAuth 2.0 standard, to confirm who you are without ever seeing your password.
• Travel booking. Comparison sites call airline and hotel APIs to show live prices and availability.
• Business software syncing. Your online store sends new orders to your accounting software, or your booking system adds clients to your CRM, through APIs.
• AI assistants. Chatbots and AI agents call APIs to look up information or take actions, and standards like the Model Context Protocol make those connections easier.

How an API request works

Most APIs you will hear about are web APIs: they work over the internet using HTTP, the same protocol your browser uses. A request has a few parts:

1. Endpoint. The web address you send the request to, such as an address for “orders” or “customers.”
2. Method. What you want to do. The most common are GET (read data), POST (create something), PUT or PATCH (update) and DELETE (remove).
3. Headers. Extra information, including authentication, which proves you are allowed to make the request.
4. Body. The data you are sending, if any, such as the details of a new order.

The server sends back a response with a status code and usually some data. Status codes are defined in the HTTP standard (RFC 9110) and grouped by their first digit:

Key takeaway: An API is a contract between 2 systems: send a request in the agreed format and you get a predictable response. You do not need to know how the other system works inside, which is why APIs let businesses connect tools quickly.

Common types of APIs

You will hear a few labels. Here is what they mean:

• REST APIs. The most common style for web APIs. REST was described by Roy Fielding in his 2000 doctoral dissertation as the architectural style behind the modern web. REST APIs use standard HTTP methods and web addresses for resources, like orders or customers.
• GraphQL APIs. Let the app ask for exactly the fields it needs in one request, instead of calling several endpoints.
• Webhooks. Reverse the direction: instead of your app asking “anything new?” over and over, the other system calls you when something happens, such as “payment received.”
• SOAP APIs. An older, more formal style still found in banking, government and enterprise systems.
• Internal, partner and public APIs. Internal APIs connect a company’s own systems. Partner APIs are shared with specific businesses. Public APIs are open to any developer who signs up.

Many APIs are documented using the OpenAPI Specification, an open standard for describing HTTP APIs in YAML or JSON. That documentation acts as the menu, and tools can generate code and test pages from it.

Why APIs matter for your business

Even if you never write code, APIs affect your tools and budget:

• Integration. Before you buy software, ask: “Does it have an API, and does it connect to the tools we already use?” A tool without a good API becomes an island of data.
• Automation. APIs power the automations that move data between apps without copy and paste, such as email automation workflows.
• Building on others. You do not need to build payments, maps or text messaging. You call an API from a provider that specializes in it, which is the idea behind embedded finance.
• Costs. Many APIs charge by usage, such as per request, per message or per transaction, often with a free tier. Check pricing models before building around one.
• Reliability. If an API your business depends on goes down or changes, your app can break. Ask vendors about uptime commitments and how they announce changes.

API security basics

Because APIs open doors between systems, they need to be locked properly:

• Treat API keys like passwords. Never put them in public code, emails or chat messages. Store them in a secrets manager or your platform’s secure settings.
• Give the least access needed. Many APIs let you create keys that can only read data, or only access certain records.
• Use standard authentication. OAuth 2.0 (RFC 6749) lets apps get limited access to an account without handling the user’s password.
• Watch for unusual traffic. Rate limits and monitoring catch misuse early.
• Remove access when it is no longer needed, including when staff or vendors leave.

These ideas fit into a broader zero trust security approach, where every request must prove it is allowed.

Where to start

If you want to see an API in action or evaluate one for your business:

1. List the tools your business uses and the data you retype between them. Those are integration opportunities.
2. Check each tool’s integrations page and API documentation. Look for clear docs, examples and an OpenAPI description.
3. Try a no-code automation tool. Many connect popular apps through their APIs without coding.
4. Ask vendors 4 questions: Is there an API? What does it cost? What are the rate limits? How are changes announced?
5. Bring in a developer for custom integrations, and make sure keys are stored securely and owned by your company.

If you are curious about how APIs fit into app development, our guide to frontend vs backend development shows where they sit in the stack.

The bottom line

An API is the waiter between 2 pieces of software: a defined way to place an order and get a predictable result, without either side needing to know how the other works inside. APIs are why your weather app knows the forecast, your checkout can take cards and your tools can share data. For a business, good APIs mean easier integrations, more automation and less manual work, as long as you secure the keys and understand the pricing. For more terms like REST, webhook and endpoint, browse our glossary.

Sources

• MDN Web Docs: HTTP response status codes — status code classes and meanings, defined by RFC 9110
• IETF RFC 9110: HTTP Semantics — HTTP methods and status codes standard
• Roy Fielding: Architectural Styles and the Design of Network-based Software Architectures (2000) — origin of the REST architectural style
• OpenAPI Initiative: What is OpenAPI — OpenAPI Specification as an open standard for describing HTTP APIs
• IETF RFC 6749: The OAuth 2.0 Authorization Framework — standard for delegated access used in sign-in and API authorization