A cycle headset attaches the forks to the frame of the bike. They have bearings and races which allow the fork to move within the frames headtube. There are a few differing headsets, older style threaded, ahead type which has a tension control from the top cap. Most use Integrated and internal headsets that use bearings that as mentioned before sit inside the headtube. Cartridge bearings can also be bought very cheaply and replaced when you get those unwanted shakes and rattles in time!