A groupset is a whole selection of the mechanical parts that are used in the make up of a bike. They include the full drivechain, bottom bracket, chain, crankset/chainset and 1 cog set. Also included are brake levers, brakes both front and rear, derailleurs front and rear, cables and wheel hubs. Some sets include discs for disk brakes and components that go with that. It is a great way to buy all the bits you need for a bike, if you have only the forks, frame and wheels.