A user agent is a computer program that interacts with a server on the World Wide Web and performs tasks associated with a particular user. The most common user agents are web browsers, which request content from web servers and display it within their user interfaces. Other user agents may include search engine robots, online chat clients, email programs, download managers, and automated testing tools.
User agents are often used in spam detection and content filtering systems in order to help identify potentially malicious requests or attempts to access restricted content. Across the Internet, user agents are sometimes used to track web usage, as each browser typically has a unique user agent.
User agents can be modified or spoofed, which is known as “user agent spoofing”. This is often done to mask the identity of the sender or to make the request appear to come from a different device or web browser. Some user agents can also be programmed with customized behavior or functionality, such as a browser extension or custom script.
User agent strings are the strings that accompany each request from a user agent and provide information such as its type and version. They are commonly formatted as per the standard guidelines specified by the World Wide Web Consortium (W3C).