When a user visits your site, the browser they use generally identifies itself with something called the "User Agent" header. This generally tells your web server which browser the user has, which version, what operating system, etc. While the User Agent doesn't always reveal the browser type, most of the time it does. This report shows you known web browsers used by your visitors. If the browser was not identified, it will fall under the "Unknown" category.
Note that MSIE refers to Microsoft Internet Explorer(tm) and Mozilla refers to the Netscape/AOL(tm) browser.