[davidrivers]

There is a defacto standard for this now as part of the HTTP protocol called "user agent strings" and all browsers send them. It's embedded in the browser itself, and it is used to negotiate content settings with the web server as Wayne said. The browser will usually identify itself, (Safari, Firefox etc) the version of HTML it supports, and the platform running the browser (Windows, IOS, etc)

For example, an ipad would send something like:

"Mozilla/5.0 (iPad; U; CPU OS 3_2_1 like Mac OS X; en-us) AppleWebKit/531.21.10 (HTML, like Gecko)"

My PC is sending "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.7 (KHTML, like Gecko) Chrome/16.0.912.63 Safari/535.7"

This is what any website can use to determine the makeup of devices and operating systems that are accessing their pages.