What is an HTTP header?

An HTTP header is a fundamental component of the Hypertext Transfer Protocol (HTTP), which serves as the foundation for data communication on the World Wide Web. HTTP headers are integral to both request and response messages, facilitating the exchange of metadata between clients and servers.

In an HTTP request, headers convey essential information about the client’s environment, preferences, and capabilities. Common request headers include "User-Agent," which identifies the client's software and operating system; "Accept," which specifies the media types that the client can process; and "Authorization," which provides credentials for authentication purposes. These headers enable servers to tailor responses based on client specifications.

Conversely, HTTP response headers communicate details from the server back to the client. Notable response headers include "Content-Type," indicating the media type of the resource being sent; "Content-Length," specifying the size of the response body in bytes; and "Set-Cookie," which allows servers to store stateful information on clients. Such headers are crucial for ensuring proper content delivery and managing session states.

HTTP headers are categorized into several types: general headers, request headers, response headers, and entity headers. General headers apply to both requests and responses but do not pertain specifically to either party's context. Request and response headers serve their respective roles in conveying pertinent information during data transactions. Entity headers provide additional context regarding the body of a message, such as its length or encoding.

The structure of an HTTP header consists of a name-value pair format, where each header field is delineated by a colon followed by a space (e.g., "Header-Name: Header-Value"). Multiple header fields may be included within a single message, allowing for rich communication between parties.

In summary, HTTP headers play an indispensable role in web communication by transmitting critical metadata that informs both clients and servers about various aspects of data exchange. Their structured format facilitates efficient processing and enhances interoperability across diverse web technologies.