yar in simple words a game engine is usually a piece of application software that provides you with and integrated development environment (IDE), a set of tools for working with rigid bodies, importing characters, animations, world building, handling physics on objects, and a way to write scripts and attaching those scripts to the objects in your game to make them work... Unity3d is a good example... give it a try...
and you should know some programming language to program your game... unity3d uses javascript, c# and boo...