This is the second part of a series of posts on Unity3D shaders, and it will focus on surface shaders. As previously mentioned, shaders are special programs written in a language called Cg / HLSL which is executed by GPUs. They are used to draw triangles of your 3D models on the screen. Shaders are, in a nutshell, the code which represents how different materials are rendered. Surface shaders are introduced in Unity3D to simplify the way developers can define the look of their materials.