A ballhead is a piece of photography gear that securely and quickly attaches a camera to a tripod while allowing the photographer to make changes to the composition quickly by simply turning one knob.
If you're researching what a ball head is, you may not be acquainted with how a professional tripod and head system works.
On cheap tripods that you see in department stores, the tripod and the head come together in one package. Generally, the head on these tripods has three handles that you twist to loosen the head and allow for repositioning of the camera.
Professional tripod systems come in two separate pieces. The legs of the tripod are only the three legs and a base with a bolt sticking out of the top. There is no practical way to use just the legs.
Coupled with the tripod legs is a head. The most popular type of head for a professional tripod is called a ballhead. This screws onto the tripod legs and is the mechanism with which the photographer attaches the camera.
Almost all ballheads have a quick release plate–a small metal piece that screws into the tripod socket on the bottom of the camera. Photographers generally leave this plate attached and never remove it. Then, when the photographer wants to put the camera on a tripod, she simply slides the quick release plate onto the ballhead, tightens down the knob, and the camera is quickly secured.
Ballheads come in many different styles, but the most common type has three knobs: (1) Securely tightens the quick release plate to keep it from sliding off, (2) A lock that prevents the ballhead from moving once the camera is in the perfect position, and (3) A knob that adjusts the tension on the ballhead so that it isn't too tight requiring extra force when positioning, but not too loose so that the perfect position is difficult to achieve.
Some ballheads have a fourth knob that allows the tripod to be panned along the X-axis (horizontally) for shooting panoramas without adjusting the tilt or height of the shot.
I have shot with most of the ballheads currently available on the market. There are some inexpensive ballheads available for under $100, but most professional ballheads cost approximately $200. After testing so many brands, however, I found some budget ballheads that are strong and which I have used for years without having any issues. You can read my tripod and ballhead recommendations here.
The real advantage of the ballhead is speed and the ability to firmly lock the camera in place. For most photography applications, the ballhead is the most popular head style. For some specific types of photography, however, other heads are appropriate. For example, wildlife photographers with large lenses usually use gimbal heads.
When shooting portraits, wildlife, night photography, landscape, architectural photography, or many other styles of photography, the ballhead is the most universal and popular choice.