macbook air and icloud

Hi

Looking to buy an air. Few questions.

A) With icloud, do all your photos get backed up? Say I copy 2 gig of pics from my pc to the mac.

B) If it doesnt because of camera roll etc, can I make it?

C) Again if not, what pictures do get backed up? How long for etc? Someone said 1000 pics for 1 month etc.

D) Do you get apps for the mac as you do for the ipad?

Hope you can help

Thanks