This is getting confusing.
Threads and topics are the same thing.
Threads/topics contain posts.
A forum consists of a tree like directory structure containing topics (commonly called threads) and inside them posts.
A post is a user submitted message enclosed into a block containing the user's details and the date and time it was submitted. Posts are contained in threads, where they appear as boxes one after another. The first post starts the thread; this may be called the TS (thread starter) or OP (original post). Posts that follow in the thread are meant to continue discussion about that post, or respond to other replies.
Members cannot delete the first post of a thread/topic.
Members may delete subsequent posts within 60 minutes of posting.
Members may delete the content of the first post if 3 characters remain. However, this is not recommended. Please contact a moderator to delete a first post.