The bolt is the part that feeds the cartridge into the chamber. It houses the firing pin and ejector, and the extractor is attached to the exterior (usually with a pin, so as to allow the extractor to grip the rim of the cartridge case).
It is an integral part of a firearm's operation; without the bolt (or an equivalent), it cannot fire.
A rotating bolt is a bolt that locks into a barrel extension via use of lugs. There is always a slot milled out for a cam pin in the bolt carrier in order to facilitate the rotating of the bolt itself as it locks into the barrel extension.