The part of a
tripod that swivels and where your
camera (or
telescope or
binoculars) is mounted: the other part is the
legset.
Tripod heads come in different kind, depending on their purpose. Heads used for video emphasize smooth movement - there is a kind called a fluid head.
Heads used for photography are either three-axis heads or ballheads.
An essential component of a worthwhile tripod head is the Quick Release System.
One good tripod head brand is Arca Swiss: another on is FOBA.
Protect your tripod head from shocks and dust. Be sure to buy one whose load capability can accomodate your camera and lens.