What are joints?

Joints - particularly hinge joints like the elbow and knee - are made up of bone, muscles, synovial fluid, cartilage, and ligaments. They're designed to bear weight and move the body. Here's how the different parts function:
Collagen: A type of tissue that serves as the scaffolding upon which everything else is built.
Tendons: They're collagen fibers that attach muscles to bones.
Ligaments: These soft tissues connect bone to bone. Joints with few or weak ligaments, like the shoulder, allow more motion (and more work for orthopedic surgeons), while joints with more supporting structures, like the elbow, are more stable, but have a smaller range of motion.
Cartilage: It gives us form before our bones are mineralized after birth - and continues to give structure to our ears and nose. In the rest of the body, it serves as the glistening plate of soft tissue at the end of bones that prevents bone-on-bone clanking. Articular cartilage (the cartilage between bones that acts as the body's shock absorber) does not have a blood supply of its own, so it needs to get nutrients from the surrounding synovial fluid.
Synovial fluid: In a normal, healthy joint, the articular cartilage is smooth, and bathed in spring water-pure synovial fluid - or joint oil. In essence, synovial fluid lubricates joints.

Joints are functional junctions between two or more bones. Joints bind the skeleton together, to give structure and allow muscles to move bones to perform certain tasks such as running, reaching and grasping. There are many types of joints the most common of which are synovial joints. These joints are freely moveable. A good example of a synovial joint is the knee, hip, elbow, shoulder, and fingers.

There are other types of joints that you are less commonly known. These include fibrous and cartilaginous joints. Fibrous joints, like that of the skull have no movement at all. Cartilaginous joints, such as those found in the pelvis have very little movement.