What it does
If you have any IP numbers listed in your moderation_keys setting (that’s where you put the spam words in the Comment Moderation setting under Discussion Options), this plugin will check them against the current visitor’s IP number. If it matches, the visitor will get a delay (that’s the tar pit — it slows them down), then an “Access Denied” message. You can customize the delay time and the message of course. Advanced users can also add additional IP matches manually, should they so desire. You might want to do that in order to match against an entire IP block, for instance.
If you’re looking for a more humorous spammer deterrent, try Kitten’s Comment Pay. The main difference here is that with “Comment Pay”, the spammer still gets to post a comment. With TarPit, the spammer cannot access any page controlled by WordPress. At all.