Game engines are programs that often used by the game developers to provide tools and can be understood as a technical core of a computer game. They enable the representation of 3D objects, effects such as explosions and reflections, the calculation of physical behavior of objects in the game, access to input devices such as mouse and keyboard, and playing music.
In the production of a computer game, either a new game engine is programmed – to the mid-1990s, this was almost always the case – or an existing one is modified, making the production time shorter.
Known commercial engines are the Unreal Engine from Epic Games, the German developer Crytek, the CryEngine and the Source Engine by Valve, known free engines are the Quake engine by id Software. There is almost always editors to game engines – programs with which you can create your own levels without professional programming skills. These are mainly for the expansion and modification of commercial games.