Suppose user X wants her account deleted. She has answered a bunch of questions and participated in several comment threads. Suppose the account is deleted; will the associated answers and comment threads disappear too?
There are two ways moderators can delete an account.
The first way leaves questions and answers having 0 or positive score intact, while automatically deleting negatively scored posts. The user's handle still appears in his posts, with a "ghost figure" replacing his Gravatar.
The second way deletes all posts and comments associated with the account. We use this option for users who are just here to spam the website with their latest product. Call it the "nuclear option."