It was seen as a major competitor to id Software's Quake series, and its technology was a little superior to the then-published Quake 1 and 2.
Since it was basically as scriptable and customizable as Quake and featured its own scripting language UnrealScript, it soon had a large community on the internet which added new modifications to change or enhance gameplay.
In its primary version (just labeled "Unreal"), the focus lay on the single-player aspect. Epic Games' 1999 followup title Unreal Tournament, however, focused mainly on multiplayer and competed against Id's Quake III Arena.
Since then, many other companies in the business have licensed the underlying "Unreal Engine" to speed up development by not building their own game engine from scratch, including the Star Trek 3rd person adventure Deep Space Nine: The Fallen or Ion Storm's Deus Ex. Rainbow Six 3: Raven Shield, however, is probably the most popular recently-released game that used the Unreal engine. Newer versions of the engine (the so called "Warfare" engine) are being used for PC games such as Running with Scissors' Postal 2, 3D Realms' Duke Nukem Forever, and Ion Storm's Deus Ex: Invisible War.
Unreal's sequel, Unreal II: The Awakening, was released in February, 2003. Unreal Tournament 2003 was released in September, 2002. It includes a Linux port on the third CD. Unreal Tournament 2004, using the next build of the Unreal engine has been announced for the first months of 2004.
There are two Wiki sites documenting the Unreal engine:
The official web site is: