The part of a building or a vehicle that is furthest from the front can be called either the back or the rear.
Rear is often used in technical descriptions. For example, car manuals talk about the rear of a car.
1. He decided to approach the house from the rear instead of going to the front door.
It is more usual to use back. For example, in ordinary conversation people talk about the back of a car, not the ‘rear‘.
2. …in the back of the dark cab, with the lights of the city streaking past.
3. Lang and Colonel von Tempelhof sat in the back.
Both words are also used as adjectives.
4. There was a rear entrance into the post office.
5. …a suitcase on the back seat.
You use back to talk about the part of an object that is at the opposite end to the front. You do not use ‘rear’.
6. He pulled his cap down more warmly over his ears and the back of his head.
7. Mimi had Carter held by the back of his trousers.
• Your back is the part of your body that is behind you, from your neck to your bottom.
• Your rear is your bottom; this is a humorous use.