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An API is a defined interface that lets one piece of software use functionality or data provided by another. A web API commonly uses HTTP: a client sends a request to a server, and the server returns a response. The API is the interface; HTTP is one protocol that can carry the interaction.
What is an API?
API stands for application programming interface. It describes a set of rules or entry points that software can use to access functionality without needing to know how that functionality is implemented. As MDN Web Docs explains, APIs make complex functionality easier to use by exposing simpler interfaces.
For a web API, imagine a browser or another program acting as a client and a remote system acting as a server. The client asks for a resource or submits data; the server handles the request and sends a response. The interface defines what the client can ask for and how to ask.
How is an API different from HTTP?
API is a broad concept: software interfaces can exist in many forms. HTTP is a communication protocol often used by web APIs. It defines how clients and servers format and exchange messages, rather than defining every capability an API offers.
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MDN describes HTTP as a client-server protocol in which a user agent, or a proxy acting for it, sends requests. HTTP is stateless between requests: each request is handled independently at the protocol level. Cookies can support sessions that preserve state across requests.
What happens in an HTTP API request?
A request has a start line, optional headers, a blank line, and an optional body. The start line identifies the method and target; headers supply metadata, and a body can carry submitted data. Here are the parts to look for:
- Method: The intended operation, such as GET to request a resource representation or POST to send data.
- Target: The path or URL identifying the resource the client is addressing.
- Headers: Metadata about the request, such as the content type or formats the client can accept.
- Body: Optional data sent with the request, often when submitting information.
GET is intended to retrieve data, not to make a change. POST is commonly used to send data. HTTP methods have different properties, including safety, idempotence, and cacheability; those properties are not interchangeable, and not every method behaves alike. See MDN’s HTTP request methods reference for method-specific details.
Illustrative request
This is a documentation-style illustration of a form submission, not a live request:
POST /users HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Content-Length: 27
name=Alex&city=LondonThe method is POST, the target is /users, and the headers describe the submitted content. The body contains the form data. The exact request structure and example are documented in MDN’s guide to HTTP messages.
What does the server send back?
The server’s response also has a start line, headers, and an optional body. The status code in its start line reports the outcome; response headers provide metadata, and the body may contain the requested or resulting data. A response need not include a body.
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For example, a client might request a resource, and the server might respond with a status and a representation in the body. The representation’s format and the meaning of particular status codes depend on the API and the HTTP rules it uses. MDN’s overview of HTTP explains the client-server exchange and response basics.
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How can you inspect web API traffic?
Use a browser’s Network panel
Open your browser’s developer tools and select the Network tab (called Network in many browsers). Reload the page or perform the action you want to understand, then select a network request. Inspect its method, URL, request headers, status code, response headers, and—when present—request or response body. This lets you observe actual traffic generated by the page without assuming what an API call contains.
Use curl from a command line
curl can send HTTP requests from a terminal and display responses, making it useful for direct inspection. The exact command depends on the API’s URL and any required headers or data. A request may fail or return an error if it needs authentication or other conditions that you have not supplied.
What else can affect an API call?
Authentication may be required before a server will provide data or accept a change. Caching can affect whether a response is reused. Cookies can help maintain session state, and cross-origin rules can affect whether browser-based code is allowed to access a response. These concerns add rules around the basic request-and-response pattern; they do not change the distinction between an API and HTTP.
In JavaScript, the Fetch API is a commonly used way to make HTTP requests. MDN’s HTTP overview provides context on Fetch and HTTP debugging. This article focuses on the shared HTTP fundamentals; it does not compare API styles such as REST, GraphQL, SOAP, or gRPC.
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