User Agent String.Com 

User Agent String explained :

Mozilla 1.0rc3
Mozilla	Name : Mozilla
5.0	Mozilla version
X11	X Window System a windowing system for bitmap displays
U	Security values: N for no security U for strong security I for weak security
Linux i586	Operating System: Linux running on a Intel Pentium CPU with no MMX support
en-US	Language Tag, indicates the language for which the client had been localized (e.g. menus and buttons in the user interface) en-US = English - United States
rv:1.0rc3	CVS Branch Tag The version of Gecko being used in the browser For the Mozilla browser this is the browser version as well
Gecko	Gecko engine inside
20020523	Build Date: the date the browser was built
All Mozilla user agent strings