API Calls 101: Understanding How the Web Communicates

If you work with modern software systems, you are making API calls all the time, even if you do not always see them directly. Mobile apps, websites, backend services, and integrations between platforms all rely on APIs to exchange data and trigger actions.

This article explains what an API call is, how it works, and why it is such a fundamental concept in software development.

What Does API Mean?

API stands for Application Programming Interface. An API defines a set of rules that allow one piece of software to communicate with another.

Instead of accessing a database or service directly, an application sends a structured request to an API and receives a structured response. This separation makes systems easier to maintain, scale, and secure.

What Is an API Call?

An API call is a request sent from a client to an API endpoint. The request asks the API to perform an action, such as fetching data, creating a record, or updating an existing resource.

In return, the API sends back a response, usually containing data, a status code, or both.

At a high level, an API call consists of:

• A destination (the API endpoint)
• A request method (such as GET or POST)
• Optional data or parameters
• A response from the server

A Simple Example of an API Call

Here is a basic example using Python and the requests library:

import requests

response = requests.get("https://api.example.com/users/123")

if response.status_code == 200:
    data = response.json()
    print(data)

In this example, the client sends a GET request to retrieve user data. The server responds with JSON data that the program can then process.

Common HTTP Methods Used in API Calls

Most APIs use HTTP as the communication protocol. The HTTP method tells the API what kind of action is being requested.

• GET: Retrieve data without modifying it
• POST: Create a new resource
• PUT: Replace an existing resource
• PATCH: Update part of a resource
• DELETE: Remove a resource

Choosing the correct method is important for both functionality and clarity.

API Endpoints and URLs

An API endpoint is a specific URL that accepts API calls.

https://api.example.com/products
https://api.example.com/products/42

Each endpoint usually corresponds to a specific resource or action. Clear endpoint design helps developers understand how to interact with the API without extensive documentation.

Request Headers and Parameters

API calls often include additional information in headers or query parameters.

Headers may include:

• Authentication tokens
• Content type (for example, JSON)
• Client metadata

Query parameters refine the request:

GET /products?category=books&limit=10

These parameters allow clients to request only the data they need.

Request Bodies

For POST, PUT, or PATCH requests, data is usually sent in the request body.

{
  "name": "Wireless Mouse",
  "price": 29.99,
  "in_stock": true
}

The API processes this data and responds with confirmation or additional information.

API Responses and Status Codes

Every API call returns a status code that indicates the result of the request.

Common status codes include:

• 200 OK: Request succeeded
• 201 Created: Resource was created
• 400 Bad Request: Invalid input
• 401 Unauthorized: Authentication failed
• 404 Not Found: Resource does not exist
• 500 Internal Server Error: Server-side issue

Understanding these codes is essential for debugging and error handling.

Authentication in API Calls

Many APIs require authentication to prevent unauthorized access. Common methods include:

• API keys
• Bearer tokens
• OAuth access tokens

Authentication details are usually passed in request headers:

Authorization: Bearer YOUR_ACCESS_TOKEN

Where API Calls Are Used

API calls are everywhere in modern systems:

• Frontend applications fetching data from backends
• Mobile apps syncing user information
• Payment systems processing transactions
• Data pipelines collecting information from third-party services

They provide a standardized way for systems to interact without tight coupling.

API Calls vs Direct Database Access

Direct database access tightly binds systems together and creates security risks. API calls provide an abstraction layer that controls access, validates input, and enforces business rules.

This approach makes it easier to update internal implementations without breaking external clients.

Conclusion

An API call is simply a structured way for software systems to talk to each other. While the concept is straightforward, it plays a critical role in building scalable, maintainable, and secure applications.

Understanding how API calls work, including endpoints, methods, and responses, is a foundational skill for any developer working with modern software systems.