Pixel shaders can be used for lighting or reflection but to generalise they are used to output the colour of pixels on a render target.

Vertex shaders process individual vertices as input by you when you make a draw call with your graphics API of choice. The vertex shader is where you perform any transformations including rotation.

You can use the CPU to perform rotation on meshes but I would recommend against it as it would increase your bandwidth use and could end up being very slow with lots of meshes.