Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322; IEMB3)
It's also known as the UserAgent string.
Mozilla/4.0 is an identifier (now old) that browsers use, it is used for browser detection. MSIE 6.0 is your browser (Microsoft Internet Explorer 6.0), Windows NT 5.1 is the identifier for Windows XP Home. The rest of the string above may be unique to Microsoft.
Modern, up-to-date browsers now use
Mozilla/5.0 as the identifier.
Other UserAgent strings will display additional information, see the one below, it references X11 (the X graphical server used with Linux), U (for United States), Linux i586 (the operating system and CPU platform), en-US which is English/United States, the "rv" information is the Gecko rendering engine version, Gecko/20070111 (using Gecko with the date of the build) and SeaMonkey/1.1 (the actual browser being used).
Mozilla/5.0 (X11; U; Linux i586; en-US; rv:1.8.1.2pre) Gecko/20070111 SeaMonkey/1.1
Hope this helps.
UserAgent: Mozilla/5.0 (X11; U; Linux i586; en-US; rv:1.8.1.2pre) Gecko/20070111 SeaMonkey/1.1