Cylinder by Starter (What is it?)

Something in the wiring was grounding out the battery on my dune buggy, and I narrowed it down to the little black cylinder thats right beside the starter. The + battery cable runs straight to it. Im pretty sure its not the starter, its the smaller cylinder next to it. Anybody know what this is? Is it a common problem, if so, is there a cheap solution? If I knew what it was I think I could go to the parts store and get another and replace it.

It is the starter solenoid.
Clark

thanks, thats all i really needed, I figure I can get another at the junkyard.