The Service Tag is Dell’s name for a serial number, and allows Dell to identify your individual machine. It should be on a sticker glued to the bottom, back or side of your computer case.
Using Internet Explorer 5.5 or higher, click on the “Find My Service Tag” link here
http://support.dell.com/support/topics/global.aspx/support/my_systems_info/details?c=us&l=en&s=gen&~tab=2
With the service tag number, you can check on the warranty of your Dell, and check the components of the original configuration of the machine, which is useful to know when you are upgrading.