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.