Blogging really is a conversation, and half the conversation is in the
But the downside of comments are that sometimes comments are spam; they aren't actually responses to your post. Spam comments attempt to profit from your traffic (and Google juice) by embedding a link to their site in a fake comment. Now, comment forms usually include a space for a link; the presence of the link itself isn't spam, but if the link goes to a commercial site selling Viagra, porn, insurance, pharmaceuticals, or a site that is purely a commerce site, it almost certainly is spam.
The presence of spam and spam comments are detrimental to your site's quality. The spam link goes to an offensive site (graphic porn is pretty common in spam links), and thus you lose legitimate readers and commenters who innocently click on the spam link.
Here's an example of a pretty typical spam comment:
Submitted by Koenraad (not verified) on Tue, 08/18/2009 - 11:33pm.
I prefer to drink coffee in the morning because it helps to be cheerful!
Spam comments tend to be short, they often show signs of being written by a non-native speaker of the language in question, they may be essentially so innocuous that they are contentless, or so out of context that they are meaningless, or downright confusing.
But the dead giveaway is the URL in the link; in the example above, the link associated with Koenraad's name goes to a penis enlargement site. If you point your cursor at the link you'd see from the URL in your Web browser's status bar (look at the very bottom of the browser window) that it goes to a penis enlargement site. You might also notice that I've used the "rel=nofollow" tag above, so that that site gets no "Google juice" from the link. And because I've added a trailing space, the address won't work without editing, either.
What can you do about spam comments? If you have admin access, delete them, otherwise, report them to your editor or admin so they can be deleted, but most of all, don't respond to spam comments. You essentially validate the spam that way, and reward them for polluting the 'net.