about diagnostic machines

i am new to the site and i have recently started my own shop, at present i have three employees. i have a major question.
does a diagonistic machine hook up to the brain of a vehicle and tell what is wrong with it. and if not, what is it called?
oh one more thing, does anyone know a ball park price of a new machine and what should i expect if i am looking for a used machine. thanks to all that reply in advance

Sheesh. If you ran your own shop I would hope you'd know what a diagnostic machine was.