Joints are connections between bones that allow for movement and flexibility in the body. There are three main types of joints: fibrous, cartilaginous, and synovial. Fibrous joints are immovable and held together by connective tissue, while cartilaginous joints allow for limited movement and are connected by cartilage. Synovial joints are the most common and movable type, featuring a fluid-filled cavity that reduces friction during movement. They include various subtypes such as hinge joints, which allow bending and straightening, and ball-and-socket joints, which enable a wide range of motion.