A joint is a connection between two structures, enabling the two parts to stay together. In the case of the body, we tend to talk about joints between two bones - the posh name for it being a synarthrosis. The connection is often built for a particular purpose, to facilitate or prevent movement as needed. For instance, the shoulder joint needs a lot of movement to allow the arm to move, whereas the bones in the skull of an adult need to be kept still without moving in order to prevent the brain getting damaged. All of the bones here are involved in joints - but they are different kinds of joints, built for a particular purpose.
There are three main types of joints connecting bones in the human body. These are fibrous joints, cartilaginous joints, and synovial joints.
A fibrous joint is a joint between two bones where the connection is made up of connective tissue - mostly collagen. It's a tough joint and most of them don't move. In fact, there are three major types.
Sutures are the type that you find between the bones of the skull. As a baby, these joints are wide - there's a bigger gap filled with collagen, which is more movable than bone, so the baby's head can squeeze out of his mother. Sutures themselves fall into one of three types - squamous, serrated or denticulate. Remember, we're not talking about the kind of suture that is used to repair an injury in A&E or surgery - this is a type of joint.
Syndesmoses are the joints that form between two bones that are slightly apart. There's an interosseous membrane between the two, made up of the collagen forming the joint, and this allows a little movement. A good example is the radius and ulna, bones in the upper limb which from a syndesmosis.
Gomphoses are 'peg-in-socket' joints. Your teeth don't (normally!) fall out after you've reached adulthood because they are joined to the jaw bone with a fibrous joint known as a gomphosis. Another way of describing the connection between a tooth and its socket is as a peridontal ligament.
Fibrous joints are often not thought of as joints because they allow so little movement, but that is the whole point - they join two bones in such a tight and strong way that they don't come apart. If the skull had more movable joints your brain would get squashed every time you put a hat on, and your teeth need to be secure, otherwise they would come out every time you bit into a slice of cake!
It won't come as a big surprise to find out that a cartilagenous joint is one that is made out of cartilage. However, to make it a bit more interesting, there is (again) more than one type of cartilage. The main thing to remember about these joints is that there is more movement than in a fibrous joint, because the joint is made up of cartilage. Having said that, it's not as movable as synovial joint.
Synchondroses are what you would call primary cartilaginous joints. These are joints of cartilage (either hyaline or fibrocartilage) between two bones. You can find them in the growth plate of a growing long bone, between the epiphysis and diaphysis (the two bits of the bone that are getting bigger). A bone gets longer because different parts of it extend; the section of cartilage in between each growing section is the growth plate, and is a great example of a synchondrosis.
Symphyses are secondary cartilaginous joints, and all of them are found in the midline of the body. The bone surfaces are covered in hyaline cartilage, but they are different from synchondroses because there is a bit of fibrous tissue in between each bit of cartilage. This ultimately makes the sections partly movable. Examples of this include the main body of the sternum and the manubrium above it; and the intervertebral discs act as the fibrous tissue between two cartilage-lined bones in a symphysis. As you may guess, the symphysis pubis in the pelvis is also a symphysis.
A synovial joint is probably what most people think of when they think of a joint in the body, because this is the type that allows all of the movement. There are four features which are important for a joint to be considered a synovial joint.
The first is that the ends must be covered in hyaline cartilage. This acts as tough shock-absorber on the end of the bone, spreading out the force so that the pressure between the two bones isn't so great.
The second is that there must be a joint cavity, a space between the two ends of hyaline cartilage to contain a small amount of synovial fluid. This is also as a shock-absorber, but also so that the bones can move in relation to each other without scratching!
There must be a joint capsule, surrounding the joint like a sleeve to keep it all together. Although every joint capsule should be made up of collagen for strength, the thickness and how tightly it wraps the joint can vary between joints.
Finally, a synovial joint requires that there is a synovial membrane. While the ends of the bone are covered in hyaline cartilage, the capsule wrapping the joint is lined on the inside by a synovial membrane which produces (and absorbs) the synovial fluid. It's got a great blood supply, with capillaries running through it, allowing it to produce more or less fluid as needed. The viscosity (or thickness) of the synovial fluid will depend on its production; the quicker the joint needs to move, the less viscous (or thinner) the fluid will be.
Two other features found in joints are discs and bursae. A disc acts to spread the load and increase how much a joint can move (e.g. in the knee). Bursae are fluid filled sacs surrounding the joint at places where things move relative to each other. It's like having a safety pad between your shoulder and a seatbelt in a car; as the seat belt moves over your should, the safety pad stops it rubbing. Bursue work in the same way to protect things from rubbing in a joint.