What is a ballhead?
At its simplest, a ballhead is case containing a metal ball with a stem that attaches to the lens mount. The ballhead attaches to your tripod and the camera (or lens) attaches to the lens mount. Knobs on the side adjust how much friction is applied to the ball, allowing the photographer to easily aim the camera exactly where he or she wants and then lock it into position. There is usually a ‘panning base’ controlled by a separate knob that allows the ballhead to rotate from side-to-side to simplify shooing panoramas. Many ballheads can also be ‘flopped’ 90 degrees to the side to change from landscape to portrait mode.
Most ballheads are similar in appearance but there are a few that are very different with things like trigger grips or handles in place of friction knobs, partial casings, extremely long stems, etc. They all serve basically the same function, though. To be honest, in most cases these look cool but as a rule don’t function any better (or even as well) as a regular ballhead. The friction-grip or pistol-grip heads, especially, are generally suitable only for light weight equipment, not for use with heavy lenses.
With the exception of the ‘very different’ heads mentioned above, the major differences among ballheads are the size (and load bearing capabilities), construction quality and materials, and type of lens mounting plate used. Minor differences include the size and type of knobs and controls, presence of a bubble level or self leveling base, and other options.