What is a game engine?

What are the responsibilities of a game engine, and which of these especially need to be efficient? +10 points?

• What are the responsibilities of a game engine, and which of these things especially need to be efficient (performance-wise)? Can I get a break-down of the purposes that a game engine is used to serve, and how it differs from a *game* itself? I'm a newb, so some intuitive things may need to be explained.... Thanks

• Answer:

  you need more then this but here are some: colision detection...(a realy smart one...and is hard to do it...) a smart loading for objects..so only objects near player are loaded... a easy key detection a vectorial system for moving the objects.. a easy categorizyng system or tags ore something so you can easy set whos enemy and who's friend or something.. some update method for each fps... tips: use as much threading as you can... use as much hastables you can...