Back & Rear, what is difference?

When I was young in middle school, I'd only learned BACK which means every BACK. 

For example, human's BACK, coin's BACK, picture's BACK.

All of BACK is BACK.

Suddenly there was REAR that was similar meaning with BACK while I was looking for a word AC Motor Cover in backside. 

"REAR"  Noun -

I. the back of a military formation or procession

infantrymen were in the rear

II. the side of an object that is opposite its front

his room was toward the rear of the hotel

Reference backside , back end

III. the part of something that is furthest from the normal viewer

it was hidden in the rear of the store

Reference back

IV. the fleshy part of the human body that you sit on

Reference buttocks , nates , arse , butt , backside , bum , buns , can , fundament , hindquarters , hind end , keister , posterior , prat , rear end , rump , stern , seat , tail , tail end , tooshie , tush , bottom , behind , derriere , fanny , ass

V. the side that goes last or is not normally seen

Reference back