From reputation section of the Stack Overflow FAQ:
2000 Edit other people’s posts, vote to approve or reject suggested edits
3000 Vote to close, reopen, or migrate any questions
10000 Vote to delete closed questions, access to moderation tools
20000 Vote to delete negatively voted answers and stronger question deletion votes
These are all "vote to delete" conditions which requires other users to agree and vote before action is taken (5 votes). Moderators ♦ are the only people that can directly and immediately delete content.
For more info, please see the Vote to Close and Moderator Tools privilege information pages.
Posts can also be removed if a post is flagged as spam or offensive repetitively.