pulley

Hello, The other day I noticed one of my pulley's is stuck. Unfortunately I don't know enough about what's what to know what it is for or what it's called. If anyone could tell me I would appreciate it. If the photos don't work, the pulley I'm talking about is directly below the alternator...

Can anyone help me identify this pulley set up, i know the top pulley isa regular supercharger set up bt what about the other two pulleys? I would like this set up for my car and i dont know where to get it