The game engine

 A game's engine is the core of the game's code. It provides the programmer with all of the features that he can use in developing the game. These include memory management, file input/output, rendering techniques, among other things.

There are a lot of engines available, some free, some commercial. Some of them simplify development by adding abstraction from code, for example Adventure Game Studio and Game Maker. Some give full power but are more complex to learn, like Open Dynamics Engine and the very popular Unreal Engine.

A custom engine can be designed too with sufficient knowledge of a programming language. Most commonly used are C++ and Python, with Java being the sole language for browser based games.