Imagine you're making furniture, you can build it entirely yourself from scratch, or you can buy something from IKEA and modify it how you want, paint it, add draws, do what ever you want, but you bought the base pre-made which saved you time, and tools required to make the IKEA part.

A game engine is similar, its the base of a game which you could create yourself manually, or you could use something else and modify it. With a lot of engines it can be as simple as "painting" it, or as complicated as using it for an entirely different genre of game.

Examples, are things like

• Source engine, which is primarily an FPS engine used for mostly FPS games
• Unity engine, used for all kinds of things, primarily made for cross-platform compatability
• Unreal engine, originally made for Unreal Tournament as an FPS engine, now expanded to be mostly a 3D environment engine, so can be used for all things from FPS/RPG games, to strategy games, and even Film creation