This document discusses game engines and provides details about Unity 3D. It defines a game engine as software designed for video game development that includes libraries and support programs. Unity 3D is introduced as a popular cross-platform game engine first released in 2005 that can be used to create both 2D and 3D games. The document outlines some advantages of Unity 3D, such as portability and an easy learning curve, as well as some disadvantages like lesser performance than other engines and limited source code access.