Unity is a game engine that was first released in 2005 and now has over 2 million registered developers. It supports development for platforms like Windows, Mac, Linux, iOS, Android, PlayStation, and Xbox. The free version of Unity has limitations like revenue caps, while the Pro version costs $1,500 per year. Installation involves downloading a 1-3GB package that expands to 3-4GB on disk. Unity uses C# and JavaScript for scripting and supports 3D formats from applications like 3DS Max and Photoshop. The engine includes tools for building applications targeting different platforms like iOS. Popular games built with Unity include Angry Bots and Epic Citadel.