uid - user identifier of the user that the process is running on behalf of
effective uid - If a program changes the uid from the uid of the executing process to it's own then it's a setuid program.
The effective uid is the uid from the setuid program - the uid stays the same
Damn that sounded confusing
Quick Google found this good page :
http://www.linux-tutorial.info/modul...ent&pageid=321