Shaders are basicly the same as textures, they are flat images.
The special thing about them is that they move or glow etc. based on what we make wait to do. Those instructions are stored in it's .shader file. The .shader file is a text file with commands that define the shader's behaviour (where should it be, how fast should it move etc. -those are onle the most basic ones but there are hundreds if not thousands of ways we can use them).
The most commonly used shaders are the ones that give Glowing effect and the ones that give Shiny effect for our skins.
Other uses of shaders are:
Creating a 'waving' effect. (banners, flags, water surface in [[mapping]])
Modifying the brush attributes (telling it to behave as a water, lava, poison or glass, making it transparent or not toutchable)
Modifying the surface attributes (make sabers or weapons not make the sign on it)