Referrer spam is when folks try to spam your referrers list in the server logs by trying to make it look like they linked to your site. This is not a big issue for most, since only you, the owner of the blog usually sees the referrers list, and so the spammer doesn’t gain any undue advantage, and yet, some people make their referrers list available publicly, and in this case, referrer spam works to the benefit of the spammer.
If you want to kill all referrer spam, or at least most of it, read Caveat Lector’s article on the topic.
While reading that topic, I came across Ann Elisabeth’s Blog, which looks like a mother-lode of information related to comment spammers, and how to trace, and kill them. I haven’t read even half of what’s on the blog, and I must it is a great resource for information.