What are UID and GID, and which UID does root have?

Question

Accepted Answer

The UID is the number that identifies a user and the GID the number that identifies a group; root is UID 0.
The kernel never works with user names. Files store the owner's UID and the group's GID, and every process runs with a UID and a set of GIDs. When the process touches a file, these numbers are compared. Names are only looked up (in /etc/passwd, /etc/group or a directory service) to show them to people.
UID 0 is special: the kernel skips the permission checks for it. Being root means having UID 0. The name root is only the usual label for it. That is why "what would make labadmin root?" has a technical answer (set its UID to 0 in /etc/passwd) and a correct one: do not do that. An extra account with UID 0 is a classic backdoor and is invisible in logs that show names. Admin rights are given with sudo (membership in the sudo group on Debian) instead, which is controlled and logged.
Typical ranges: 0 for root, 1–999 for system and service accounts, 1000 and up for human users.
Go deeper:

Wikipedia: User identifier — UID ranges, real vs effective UID, and why 0 is special.
 Wikipedia: Group identifier — how GIDs and supplementary groups work.