Where is a user agent string used?
The user string is sent along by the browser when a visitor goes to a website. The website programmer can then program exceptions, which allow the website to load best in a particular version of a browser. Just imagine that the visitor is using an older browser, in which case an alternative version of the website can be loaded for better viewing.
A user agent parser is used to read out the variables from the user agent string. These variables are divided in the following categories: browser type, browser version, operating system.