KVM Switch for Desktop/Home server set up

[N4g4rok]

Possibly a silly question, but i've only seen a KVM switch setup once, and didn't really understand how it worked. I've got two desktop systems, and i would like them to be able to share a keyboard, mouse, and a DVI monitor. I see KVM switches on Newegg going from $20 to $80 and well above. What should i be looking for?