UUID stands for Universally Unique Identifier. It looks like 8-4-4-4-12 hex characters separated by hyphens, a 128-bit number written in a standard layout.
The point is to generate IDs that won't collide with anyone else's, ever, without asking a central server. That makes UUIDs perfect for distributed systems, offline apps, and database primary keys.
Different versions trade off randomness vs sortability, see the related glossary entries below.