A user agent is a software application that acts on behalf of a user to interact with web servers and access online content. Most commonly, this term refers to web browsers, but it can also include other tools like search engine crawlers or mobile apps. When a user agent requests a web page, it sends a string known as the user agent string to the server, identifying itself and providing details about the device, operating system, and browser version. This information helps websites deliver content optimized for different platforms and devices.