View source for Uid

UID stands for USER ID, it is a number usually starting at 0 (for [[root]]) and ending at 65535.  It can go beyond in some unix like operating systems.

Every user has a UID when they have a shell on a system.  The 'id' command lets you see your USER ID

 id
 uid=1000(pjp) gid=1000(pjp) groups=1000(pjp), 0(wheel), 553(_saned)

In this case the User ID is 1000.  The file that provides a mapping of username to user id is /etc/passwd and /etc/master.passwd in a BSD system (/etc/shadow on SVR4).  Some
programs check the mapping between user and userid as in this case:

 $ su
 su: who are you?
 francisco$ id
 uid=1000 gid=1000(pjp) groups=1000(pjp), 0(wheel), 553(_saned)

Notice id doesn't map the userid to a username since I changed it in /etc/passwd.