Most tripod heads are 'fluid' heads. They have an internal liquid which creates a dampening effect known as drag. When operating a tripod it is important that you have some control over the level of drag - entry-level heads will sometimes have no control, whereas professional range heads have finer control and will allow you to determine the level of drag. The control can either be in the form of a dial, where you dial in the amount of fluid, or with a stepped fluid head predefined stages will be the same every time you use the setting. The stepped fluid gives you the best ease of control.
Essential Tripod Handling
When checking a tripod head, make sure that there is no 'play' in it, i.e. when you begin a movement, there should be no jerkiness or inconsistencies in the drag; it should start off with an instantly smooth motion. Good tripod heads will also offer a quick release plate for transferring your camera to handheld in a matter of seconds.
Stable Balanced Tripods
Some tripods have a counterbalance system to help keep the camera balanced. There are a couple of benefits to counterbalance; you can position the camera further forwards or backwards on the head, and a system of springs in the head will help to keep the camera neutralised in a horizontal position.