This is more of brief on blog spam rather than a full blown run down on the whole thing. One thing is for sure and that is if you have a blog, after a while you are going to get spam comments and you could get a lot of it.
Why do you get blog spam? One of the main reasons you get it is because people are trying to build up back-links to their blog site or web page. They want to do this because it makes their web site or blog rank higher with Google and there is nothing wrong with this in itself, back-links are good (back-links are where you put a link back to a website as part of a blog post comment in this case). The problem comes when people start making comments that have nothing to do with your web site or blog post and they are doing it merely for the link.