The tripod is a three legged support system built to hold cameras, telescopes, etc higher up so you don't have to hold them yourself.
No not usually. Each manufacturer has a different mount and many cheaper tripods make you screw directly into the camera. A good tripod is normally two parts, the legs and the mount and can be sold...
He Works At The Krusty Krab At Bikini Bottom, Ehe I Know This Cause Im His Mum (: