UUID v1: Time-based UUIDs that include MAC address and timestamp. Useful when you need sortable, unique identifiers.

UUID v4: Random UUIDs using cryptographically secure random numbers. Most commonly used version for general purposes.

UUID v5: Namespace-based UUIDs using SHA-1 hashing. Deterministic - same input always produces same UUID.

Use Cases: Database primary keys, API tokens, session IDs, distributed systems, and anywhere you need globally unique identifiers.