Unity is a cross-platform game engine developed by Unity Technologies, which is primarily used to develop video games and simulations for computers, consoles, and mobile devices. It can be used to build 2D, 3D, AR, and VR apps and games. It allows you to import and assemble assets, write code to interact with your objects, create or import animations for use with an advanced animation system, and much more.
This Tutorial firstly talks about the basic structure of Unity. It then is a step-by-step tutorial, on building a basic game using Unity.