When an account is deleted, posts made by that account on the main site with a negative score are deleted along with it. (Posts with a score ≥ 0 are preserved.) Previously all negatively scored posts on the meta site were also deleted, but this behaviour changed very recently and now all meta posts are preserved. All comments are preserved, although like posts they are "anonymised" and attributed to a "user#####".
(For completeness, there is a stronger form of account deletion, called destruction, which will delete all contributions by the account. This form is generally reserved for spammers and users trolling the site — so "contributions" is perhaps an overstatement — and additionally feeds into a system designed to prevent those users/people/bots from posting again.)