Anyone can delete their own post if it is within the 4 hour edit window. You do this by hitting edit, then deleting everything in the text body, and the subject line, and type "delete" in the subject line.  The only other way posts get deleted is by admin. Either admin sees something when checking the boards, that does not meet the guidelines, so they remove the post, or someone hits the 'report a post' button, and admin will look at it and delete it if it doesn't meet guidelines. When a post is deleted, all replies go with it, so sometimes entire threads, or entire 'branches' of a thread disappear.