What a Minecraft UUID is and how to look one up
A Minecraft UUID is the permanent identifier the game gives every Java account. It is a 32 character hexadecimal value that never changes, even when a player renames their account. Usernames come and go, but the UUID underneath stays the same, which is why servers, plugins, and databases all track players by UUID rather than by name.
To look one up here, switch to UUID lookup and paste either a Java username or a UUID. If you paste a username, the tool resolves it to the account behind it and shows the public Java profile, including the current name, the UUID in both formats, and the player skin. If you paste a UUID, it works in reverse and returns the profile that owns it. Everything runs against Mojang's public profile service, so no login is required and nothing you enter is stored.
UUIDs are written two ways. The trimmed form is 32 characters with no separators, and the hyphenated form splits it into five groups of 8-4-4-4-12. They point to the exact same account, but different tools expect different shapes. Server files like whitelist.json and ops.json use the hyphenated form, while many plugin configs and APIs accept either. The lookup shows both, so you can copy whichever one your setup needs.
This matters most for people running servers. When you whitelist a player, ban an account, assign a rank, or grant a permission, keying off the UUID means the assignment survives a name change. If you only stored the username, a player who renamed, or a stranger who later claimed that freed up name, could end up with the wrong access. Looking up the UUID first is the safe way to reference a specific person.
It is also the quickest way to answer everyday questions. Find the current username behind an old UUID from a log file, confirm that a UUID really belongs to the player you think it does, or grab someone's skin from their profile. Because the result is the live public profile, the name it shows is always current, not whatever the player was called last week.
One thing a UUID lookup cannot tell you is whether a username is free to claim. A UUID always belongs to an existing account, so if you want to know whether a name is available, use the main username checker instead. The UUID tools here are for identifying and inspecting accounts that already exist.