we're going to need more details than that. "i don't understand" doesn't give us anything to go on. what don't you understand? how the backups work? how to set it up on a Mac? what photo stream is? iCloud has many aspects, so what exactly are you confused about?
well after you set all settings in the icloud section on your ios devices and make your account on the icloud website everything should be in sync whenever you make a change on one device