Joints are the moveable connections between two bones.  Joints are made up of joint capsules, a joint cavity and the surfaces of the bones. 

The bones are covered with cartilage, which acts as a protective layer to stop the bones in the joints from rubbing against each other.  It also helps to transfer any pressure from the joint to the bone supporting it. 

The bones of the joint are separated by a thin space that is enclosed by the joint capsule to make it airtight.  The joint capsule includes an protective outer fibrous layer and an inner layer with nerves and blood supply.

The space inside the joint capsule is called the joint cavity.  The joint cavity contains synovial fluid.  Synovial fluid, produced by cells inside the joint capsule,  helps to make the joint move smoothly and acts to disperse any pressure put on the joint.  Synovial fluid also nourishes cartilage.