Essentially, comment spam is a blog comment that's nonsensical, off-topic, or complimentary in a way that adds no value to the topic or the discussion (like "great post" or "nice blog"). Most importantly, however, it always includes a link to another website. Here's one I received today, just as an example:
Anonymous has left a new comment on your post "International shipping scams, part 3: out of the nutshell":Well I, too, am glad you came across this in your hunt for something concerning this.
I'm impressed, I have to admit. Seldom do I come across a blog that's both educative and entertaining, and without a doubt, you have hit the nail on the head. The problem is something not enough men and women are speaking intelligently about.
I'm very happy that I came across this in my hunt for something concerning this.
Feel free to surf to my homepage - Instant income blog [actual hyperlink removed]
In most cases, comment spammers are trying to scam search engines into boosting a website's ranking by sprinkling links to it all over the Internet. Many search engines use the number of inbound links to a particular website as an indicator of that site's popularity and credibility, and the more external links there are to a given website, the higher up in search results it will appear.
In other cases, comment spammers are trying to drive traffic to a particular website by providing convenient links to it everywhere they can. The links might lead to commercial websites, they might lead to porn websites, they might lead to malicious websites that put nasty files and programs on the computer of anyone who visits them.
Some bloggers are philosophically opposed to moderating the comments they receive because the spirit of the blogosphere is open discussion in which anyone and everyone can participate, share ideas, and debate points of view. They rely instead on filter programs to identify and block spam. But filters have their limits: some spam comments are not blocked, and some legitimate comments are inappropriately blocked.
My choice is to moderate all the comments I receive. I don't publish any comments that contain links I haven't tested myself first to make sure they're legitimate and safe.
In the process of doing a little research on this, I came across a couple resources that are worth checking out. One is an article titled 5 Ways to Identify Blog Comment Spam on About.com that discusses how people who use spam filters for their blogs can catch any comment spam that their filters miss.
Another is the Museum of Comment Spam, a hilarious collection of comment spam messages, "lovingly curated" by a witty blogger named Don Brown. Ditto for Michelle Kay's post titled 12 Most Silliest Spam Blog Comments That Made Me LOL! I think you'll be happy you came across this in your hunt for something concerning this.