The Appendicular Skeleton refers to your arms and legs so called because they append, that is attach to, the main body structure.
These appendicular bones attach to the main body with girdles, which gives them an extraordinary range of motion. In fact, the joints are a key part of the effectiveness of our limbs, and are largely responsible for the way we move.
For example, the upper arm attaches to the body via the pectoral, or shoulder, girdle. This joint also includes the scapula and the clavicle. This is a ball and socket joint, which allows the upper arm a huge freedom of movement from the upper body.
The upper arm (humerus) connects to the forearm (two bones the radius and the ulna) with a hinge joint. Having two bones in the forearm, rather than one, allows for a wide range of twisting at the wrists.
If we look at the lower limbs we again find a ball and socket joint at the hip. Into this insets the femur. The femur is longest, strongest, largest bone in the body, allowing it to withstand the tremendous forces we exert on it the act of jumping hard, for example can place a load of up to two tons of pressure per square inch on the femur.