Comment Karma is used to filter spam. The comment is given karma "points" based on certain criteria. (By the way, I absolutely recommend the SpamKarma plugin for Wordpress...it's simply awesome for spam management).
Comment agent I believe lists the kind of browser is used to post the comment (or if it's a bot). Again, another feature to prevent spam.
Type is whether it's a trackback or an actual comment.
I'm not sure what the POSTS table stuff is.
I hope this helps!
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