Headers are elements used in web development and document formatting to define and organize sections of content. In HTML, headers are represented by tags such as <h1> through <h6>, with <h1> indicating the most important heading and <h6> the least. Proper use of headers improves readability, helps users navigate content, and enhances search engine optimization (SEO) by signaling the structure and hierarchy of information to both users and search engines. In other contexts, such as HTTP, headers refer to metadata sent with requests and responses, providing essential information about the communication between clients and servers.