Remembering the past to build the future
If you are new to Unity, or you have only used Unity 5.x, you can skip this section or read it just for curiosity.
In Unity 4.x (before version 4.6) and other previous versions, building 2D games was a bit harder. In fact, you needed to use a range of different methods and tricks to achieve the illusion on 2D. All the 2D objects were actually 3D objects viewed in a particular perspective or with a particular camera, which gave the illusion of a 2D object.
From Unity 4.6 on, and especially since Unity 5.x, this is not needed any more. There is built-in support for 2D games. So now, there are special components to deal with 2D objects, and the following sections will explore some of them.