A joint (or articulation) is the point where two or more bones meet, with muscles acting on them to produce movement — though not every joint is actually movable.
The chapter classifies joints in two independent ways. The FUNCTIONAL classification looks at how much movement a joint allows: synarthroses are immovable joints, amphiarthroses allow slight movement, and diarthroses are freely movable. Freely movable joints are concentrated in the limbs, where mobility matters most, while immovable/slightly movable joints are concentrated in the axial skeleton, where protecting internal organs and providing a firm, stable attachment matters more than mobility.
The STRUCTURAL classification looks at what actually separates the bones at the joint: fibrous joints (bones united by fibrous tissue — generally immovable, further split into sutures of the skull, syndesmoses like the radius-ulna membrane, and gomphoses like a tooth in its socket), cartilaginous joints (bones united by cartilage — only very slight movement, e.g. between the ribs and the sternum), and synovial joints (freely movable, cushioned by synovial fluid, built from an articular cartilage, articular capsule, joint cavity and reinforcing ligament).
Synovial joints — the freely movable kind found mainly in the limbs — are further divided by the SHAPE of their articulating surfaces into six types: ball-and-socket (hip, shoulder — the greatest range of movement, including rotation), hinge (elbow, knee — back-and-forth flexion/extension in one plane only), pivot (the joint between the first two neck vertebrae — rotation around a single axis, as when turning the head), gliding/plane (the wrist's intercarpal joints — flat surfaces sliding past each other, no rotation), saddle (the thumb joint — rocks back-forth and side-to-side with limited rotation), and condyloid (the wrist/knuckle joints — side-to-side and back-forth movement on two axes, but no rotation).
This classification also underlies basic movement vocabulary used across the chapter and in sport generally — a FLEXOR is a muscle that decreases the angle at a joint (flexion), which is exactly the kind of back-and-forth movement a hinge joint like the elbow or knee is built for.