A user agent is a string of code used by a user's web browser to identify itself to a web server. User agents are typically sent as part of the HTTP requests and provide information about the user’s browser and operating system.
Web servers typically use user agents to offer tailored content to users based on the capabilities of their browsers and operating systems. For example, a web server might use the information provided by a user agent to serve different versions of a web page to users with other browsers.
User agents are also used by web analytics tools to collect information about the browsers and operating systems that users are using to access a website. This information can be useful for optimizing the user experience.