If you've been on the internet for any amount of time you're probably familiar with "spam" in your email inbox. For the uninitiated, spam is an unsolicited commercial message, or something you didn't ask for trying to sell you something.

So what does this have to do with blogs? Well just like you can get spam messages in your inbox, people will leave spam comments on your blog. However unlike email spam where the target is you, comment spam generally targets search engines.

Comment Spam and Search Engines
Why on earth would a spammer target a search engine on your blog? Let's start from the beginning. Several years ago Google pioneered a search technique called PageRank. Basically what it does is, in addition to looking at the content of a page they index, they also look at who links to a page and what that link says. This technology is what made Google very good at returning relevant results and made it the most popular search engine today. Because their ranking system relies so heavily on PageRank people can sometimes game the system in what's called "Google Bombing."

A google bomb is when a large number of different websites link to a page with the same link text to influence the ranking of that page for a search term.

This brings us back to the spammers. A spammer might have a site that sells viagra and wants to be at the top of a search for viagra on Google, so to create the effect of a google bomb they leave comments on hundreds or thousands of weblogs linking to their site with the link text "viagra". They don't really care if you see it, in fact they'd rather you didn't because you would delete it, they just want the search engine to see it when they index your page.

Fighting Comment Spam
Comment Moderation is very effective in addressing unwanted comments. The best defense against comment spam is just watching your comments. Under Manage → Comments it shows a listing of the latest comments on any post and you can quickly scan the comment activity on your site. The faster you respond to comment spam on your site, the less likely the spammers will return.

On the Combating Comment Spam page you will find a list of more proactive measures against comment spam, including links to helpful plugins.

Stealth Spam
A new technique is the spammers will leave a perfectly normal-looking comment except for the commenter's URI or name. The best way to watch out for this is to visit the URIs of people who leave comments on your blog. (This is a good practice anyway.) If one looks suspicious, either delete the comment entirely or leave the comment and delete the URI.

Another way of stealth is to use a div-tag around a bundle of hundreds of links. This becomes more and more common because many software displays directly the given HTML tags and not the HTML code. To avoid this the software must "strip-out", other word: filter the HTML tags while inserting the comment into the database.

Combating Comment Spam
Comment spam is a fact of life if you have a blog. Using WordPress, you have not only solid built-in tools to prevent comment spam, there are also a wide range of comment spam protection and defense plugins and methods to choose from if you feel you need additional coverage and protection.

There is no "one size fits all" method that will protect your comments; spammers use many tactics. Consider using multiple defenses. Remember spammers change the way they attack so you must keep your choices updated.

Akismet Comment Spam Fighter
Current versions of WordPress come with Akismet installed by default. Akismet uses a unique algorithm combined with a community-created database to "learn" which comments are comment spam and which are legitimate.

To enable Akismet on your WordPress blog, go to the Plugins panel and activate the Plugin. A menu is added to the Comments Panel and holds a list of "caught" comment spam.

If comment spam gets through Akismet's net, mark it as comment spam in your Comments Panel. Do not delete it. By marking it "comment spam", the information is sent to Akismet and added to the community-created database.

Frequently, check through the caught comment spam in the Akismet Panel to look for false/negatives, legitimate comment spam that has been caught by Akismet. Mark it as Not Spam and click Despam at the bottom of the page to remove these comments from the list.