Comment spam is where a scumbag spammer automates(?) a program to search out weblogs that allow comments and add a comment that links to their website. The website is trying to sell something of dubious value, of course. Some are creative enough to even make the comment look relevant to the post they are “replying” to (which is why I question the automated part of the equation). Their goal is primarily to increase their Google PageRank and secondarily to drive traffic.
I just received my first comment spam - some people are inundated by the stuff. Note to spammers - don’t waste your time…you won’t get much of a kick from my PageRank.