ZigTrap adds a hidden field to your comments form, which humans will leave empty because they don’t know it’s there. Spam bots, however, will normally enter some content in it, and this springs the trap, causing WordPress to show a message and not save the comment at all.
This is known as the “honey trap” or “honey pot” technique of spam prevention. Use this plugin in conjunction with Akismet for a pretty solid spam prevention package. I use it in place of a CAPTCHA test, (CAPTCHAs are a usability disaster and universally hated).
Please ensure that the comments form in your theme calls the ‘comment_form’ action.