For small, lightweight cameras, one may use a tripod where the head (generally guaranteed to be a friction head) is permanently attached and forego reading the next few paragraphs altogether.
Anyway, friction heads employ fiber discs rubbing against metal washers to create resistance when panning (lateral movement along the x axis) and tilting (vertical movement along the y axis). They are most commonly found standard on smaller, lightweight consumer tripods and are not removable. Friction heads also have the worst reputation for smoothness and reliability.
Fluid heads work by using "high-viscosity liquid" (god knows what, exactly... -my bet is on a hybrid corn syrup/motor oil concoction ) that is forced through grooves inside a metal drum. Whereas with a friction head one can usually only adjust resistance by loosening or tightening the locking mechanism (or separate controls that simply squeeze the friction discs closer together, either way, not terribly precise nor reliable), a fluid head's tension can be "dialled up" by adjusting how much fluid is squeezed through the grooves (is it getting hot in here, or what?), thereby posing no danger of wear to the pan and tilt mechanisms.
Geared heads, generally considered the smoothest and most precisely controllable of all the types of heads, employ manual wheels connected to interior gear assemblies which drive the lateral and vertical movements. Each wheel (one on the rear for tilt, one on the left for pan) has its own choice of, usually three, gear settings (for tension) as well as a neutral gear. In first gear it may take, say, five full 360-degree cranks of the wheel to achieve your desired pan; in third gear the same move would take two cranks (approximately... -I don't remember the exact ratio, but you get the idea). Geared heads are not very intuitive or user-friendly like a fluid head, and it takes a lot of practice to get the hang of them. You need to use both hands to operate the wheels, and by design the tilt wheel's orientation moves left-right, and the pan wheel (in your left hand, in front of you) moves forward-back or up-down, which all gets terribly confusing when you factor in clockwise-counterclockwise motion as well. And for high-speed, whip-pans, you're better off sticking to a fluid head. Practice operating a geared head without a camera mounted and try to achieve perfect circles and figure-eights... trust me, it's hard.
Photos from the front lines:
O'Connor 25-75 fluid head
Arri 2 geared head
Chapman PeeWee dolly
