A UUID is a 128-bit identifier you can create anywhere without asking a central authority, which is why they end up as database keys, request identifiers, file names and idempotency tokens. This tool makes them in bulk, in either of the two versions worth using today: v4, which is entirely random, and v7, which puts a timestamp in the leading bits so the values sort in creation order — a real advantage for database primary keys, where random ones scatter writes across the index. The randomness comes from your browser's cryptographic generator, and nothing is sent anywhere, so the values are yours alone.
Yes. They come from the browser's cryptographically secure random generator, in the page. Nothing is requested from or reported to a server, so no one else has ever seen the values you generate.
In theory yes, in practice no. A v4 UUID has 122 random bits; you would need to generate billions per second for decades before a collision became likely. It is safe to treat them as unique.
Whenever the identifier becomes a database primary key. v7 starts with a millisecond timestamp, so new rows land next to each other in the index instead of scattering across it — that keeps inserts fast and the index compact. Use v4 when the value must reveal nothing at all, since v7 discloses roughly when it was created.
A v4 UUID is random enough to be unguessable, so it can serve as an opaque identifier in a URL. It is not a substitute for a real session token or API key, which should be longer and issued by your authentication system.
Version 1 encodes the machine's MAC address and leaks where it was made. Versions 3 and 5 are hashes of a name and are only useful when you need the same input to always produce the same identifier. For general use, v4 and v7 are the ones to reach for.