Which type of posts should I report?

[e1ghtSpace]

Which posts are you supposed to report and which posts are you supposed to leave?

[Quickseller]

Any post that (you think) breaks the rules. You should also report the OP of a thread that is in the incorrect category (for example a thread trying to borrow money listed in the Bitcoin discussion section.

All that reporting a post will do is have a moderator look at the post/thread and will act accordingly.

[Dare]

You should report posts that you think are spam or break the rules and threads that are in the wrong category. Reporting insubstantial necroposts is also advisable, but that's more of a personal opinion than forum policy. For more specifics on the forum's rules, take a look at this thread: https://bitcointalk.org/index.php?topic=703657.0

[Vod]

I just reported four posts for copyright theft, and they all got deleted.

Copyright theft is when you cut and paste an entire article.  By law, you can copy only one or two paragraphs, as long as you link back to the source.