An HTTP header is a key–value pair included in a request or response that provides metadata about the communication. Headers control authentication, caching, content type, compression, cookies, and more. Both clients and servers use headers to negotiate how data should be handled. Common headers include Content-Type, Authorization, and Cache-Control.
Why it matters
Headers are central to how HTTP works. They define behavior for APIs, browsers, caching systems, and security layers. Misconfigured headers can lead to security vulnerabilities, broken requests, or performance issues.
Examples
A response header like Content-Type: application/json, or a request header with Authorization: Bearer <token>. Lessons like Headers and Metadata break down common header types.