The user agent string, a piece of data transmitted in the HTTP header during a web request, contains information valuable in determining browser type and often basic system information.
Example user agent string sent from a web browser during an HTTP request:
Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US) AppleWebKit/532.5 (KHTML, like Gecko) Chrome/22.214.171.124 Safari/532.5
The above example, for instance, provides information such as browser and browser version, user locale (language), OS, system architecture and the layout engine used. When authoring documents for the Web, information from the user agent string can be valuable in determining how best to mark-up documents.
Getting the information is easy.