The Unreal Tournament (or short: UT) series consists currently of the following five games:
All of them are first person arena shooters. The first title of the series was an offshoot of the Unreal game. It came completely without a single player campaign, which was a novelty back then. It was primary in competition with the Quake 3 Arena. It's successor, Unreal Tournament 2003 was received well, but was put in the shade soon after, when Unreal Tournament 2004 was released.
The title Unreal Tournament 3 was not as big of a success as his predecessors. Most players went back to UT2004 after a while or transitioned to other games.
As of now (January 2022) UT4 is not yet officially released and may never be. Epic Games stated, that they put the project on ice in favor of their Fortnite title.