What makes an arm an arm and a not a leg?

An arm is for changing things around us or climbing.
A leg is a limb that we walk on. Legs are stronger and not as moveable as arms.

cats have legs that are all made for wallking on. Leg muscles are for jumping and balancing.

Apes use their arms for balancing as well as their legs. But they are arms, not legs because they use their arms more often for climbing. That is why their arms are not called legs.

Humans have both arms and legs. They are designed to stand upright and change things with their arms and hands.