Acronym for User ID.

The numerical id that corresponds to a user. For example 0 is usually root. bobs UID might be 500. UID to name mappings are stored in /etc/passwd (see passwd(5))

A program is said to be "SUID" when the file has the "+s" user permission bit set, so the program will run as the permissions of the user that owned the file, not the user that ran the file.
Actual UID
The uid listed in your auth mechanism against the username you logged in as. Alternatively, the uid a process was launched by, as opposed to the uid it runs as (the EUID)
Effective UID (EUID)
The user that a programs permission checks are done against. This is often root if you run a SUID program. The Actual UID is your normal UID, but the effective UID is that of root.