Ive been bugged for awhile now on my inability to find an answer that suits me on this. What makes an arm an arm, and not a leg. If you say hands and feet, Please tell me how many arms or legs monkeys have!
(google has failed me on this silly queary)
In people, at least, legs are clearly designed to deal with the constact impact of our weight falling on them, where our arms are not. If I'm not mistaken this also holds true, if to a lesser extent, with monkeys and the like.
My layman's definition:
Legs are for walking.
Arms are for holding.
Not binary, but a sliding scale. Legs are used for locomotion primarily, and manipulation is secondary. Arms are the inverse. At least that works for most land mammals that I can think of.