Taking a break, or leaving for good

These are two different things, and only one of them can be undone. Deactivating is a pause, and it is reversible. It hides you (you become undiscoverable, you are hidden from other people's orbits, and you appear offline), and it signs you out everywhere. Nothing is deleted. Signing back in with your password reactivates your account and puts your privacy choices back exactly as they were — you never come back more visible than you left. It is in Settings → Account → Deactivate. Deleting is permanent. It removes your account, your orbit, your posts, your messages and your media. Photos and videos you own stop being shown straight away and are then deleted from storage; where another person had saved one of them, they see a "no longer available" note in place of the picture. Two honest exceptions: a picture that somebody else's own records also claim as theirs is left alone rather than guessed at, and a profile photo that was copied onto a friend's contact entry is not a stored file and stays with them. Some things are deliberately kept: communities you ran are handed to someone else rather than destroyed, and other people keep their own private notes about you in their own orbit. Before deleting, you are offered a copy of your data to download. Take it if you want it — after deletion it cannot be recreated.