>>264321426
TL;DR: Shaders are programs executed by your GPU.

They can be anything from physics simulations to entire rendering pipelines. Most are just programs that tell your GPU where objects should be located relative to the camera viewpoint, and what they should look like. That being said, you can do all kinds of fancy shit with them, since you are getting into the code that determines exactly how/why/what something looks like. You can be very creative, there are almost no limits to how you can instruct the GPU to interpret the data you give it and what you tell it to do with it. You can even write malicious shader programs that just shut down people's computers.