Joints are a component of an endoskeletal system, which allows for flexibility and movement. There are four different primary joints located throughout the body. These are the hinge, the ball and socket, the picot, and the glide joints. The each serve different purposes, and are located in areas best suited for those purposes.
The hinge joint is basically a hinge. It can move up and down or from side to side, but within a 180o range. A very common example of this is a person's elbow or knee. Hinge joints usually have a lot of stress placed on them, and the knee is a very common example of this. Knee surgery is more common than any other surgery performed on a joint, because they are used rigorously each day. They are the joints that move you up, down, forward, and they have to carry all of your weight. This stresses the knee joint so much that it is not uncommon for even an average person to damage them in some way.
Ball and Socket
The ball and socket joint allows for increased range of movement. The best examples of this type of joint are the shoulder and hip joints. Even though the shoulder is considered a ball and socket joint, the hip is considered the most "true" ball and socket joint because the ball is almost completely enclosed by the socket. The shoulder is more like a ball resting inside of a shallow dip, rather than being fully encompassed. This does, however, offer increased mobility and range, at the cost of being easier to pop out of joint or being injured. Shoulder problems are very common because of this joint structure. In fact, 13.7 people in the United States had treatment for shoulder-related problems in 2003.
The pivot joint allows for the most movement out of all of the joints. The most common example of this joint is the neck, which can look from side to side, up and down, and almost any way at all. This joint is used primarily to be able to see your surroundings without turning your whole body. This helps when you are driving or carrying large objects, but other than that, there is little else that it can do.
The glide joint is similar to the ball and socket joint in terms of the range of movement, but there is no actual ball and socket. An excellent example of this is a person's wrist, which has extreme mobility but no bulky ball and socket like the shoulder. Glide joints are also very delicate, and can be easily damaged. They are also very weak, and cannot apply very much force by themselves. For example, try lifting a 30 lb weight with your whole arm, and then try lifting the same weight using just your wrist. Gliding joints are useful, but lack a lot of support that allows other joints to apply more force.