A user agent is a brief passage of text that attempts to describe the software or browser (the “Agent”) ending the request to the website. In the requests they send to websites, web browsers contain the user agent string. A description of the operating system and device type that the browser/agent is using is frequently included in the user agent.
User Agents usually look quite strange
User agents aren’t meant to be viewed or used by “regular” internet users, and the formats of many widely used user agents are the result of a lengthy and complex past.
User Agents tell a website what browser you are using
So every time you load a web page, image, video, or song in your web browser, your browser makes a “request” for that file. Included in that request is the user agent (In a HTTP Header). Web sites can look at that user agent string and determine what web browser, operating system, and device you are using.