When a user is _deleted_, all questions/answers _except_ downvoted ones are kept. 

When a user is _destroyed_, the account and all posts are deleted. This is usually for spammers.

The votes get removed in both cases, and affected users will see a `-xyz User was removed` entry in their reputation history.

See also: http://meta.stackoverflow.com/questions/125740/dont-throw-away-all-votes-when-a-user-is-deleted