You choose to either deactivate your account (temporary) or to delete it permanently. Here's what Facebook says about deleting it.
How do I permanently delete my account?
If you don't think you'll use Facebook again, you can request to have your account permanently deleted. Please keep in mind that you won't be able to reactivate your account or retrieve anything you've added. Before you do this, you may want to download a copy of your info
from Facebook. Then, if you'd like your account permanently deleted with no option for recovery, log into your account and let us know
When you delete your account, people won't be able to see it on Facebook. It may take up to 90 days to delete all of the things you've posted, like your photos, status updates or other data stored in backup systems. While we are deleting this information, it is inaccessible to other people using Facebook.
Some of the things you do on Facebook aren't stored in your account. For example, a friend may still have messages from you even after you delete your account. That information remains after you delete your account.
To get to this on Facebook, log-in, then go to your settings, at the top right corner, click on the little arrow for the drop down menu, then choose "Settings", click on "Support Dashboard" along the left side, click on "Help Center", scroll down (along the left side) to "manage your account", finally go to the Deactivating and deleting account link and click on the underlined phrase "let us know" at the end of the 1st paragraph and where you can actually click to have it deleted.