1. Spam in blogs (also called simply blog spam or comment spam) is a form of spamdexing. It is done by automatically posting random comments or promoting commercial services to blogs, wikis, guestbooks, or other publicly accessible online discussion boards. Any web application that accepts and displays hyperlinks submitted by visitors may be a target.

     Adding links that point to the spammer’s web site artificially increases the site’s search engine ranking. An increased ranking often results in the spammer’s commercial site being listed ahead of other sites for certain searches, increasing the number of potential visitors and paying customers.

     From: http://en.wikipedia.org/wiki/Comment_spam

2. Comment spam

      Comment spam: Comments made with the sole purpose of gaining a backlink, which in turn will send traffic or link juice (provided the blog in question doesn’t use the no follow tag) to the site of the spammer. Some spam comments are very explicit and come with several links to drug or gambling related sites. Other forms are more subtle, where the commenter actually tries to leave a relevant message on the blog.

From: http://aris-haryana.blogspot.com/2008/11/bloggers-glossary-part-2.html

3.   Often blog comments include URLs, as a result spammers will often post blog comments in an effort to boost the incoming links to their websites. Comment spam often make no sense and the comments are simply a product plug. : Comments made with the sole purpose of gaining a backlink, which in turn will send traffic or link juice (provided the blog in question doesn’t use the no follow tag) to the site of the spammer. Some spam comments are very explicit and come with several links to drug or gambling related sites. Other forms are more subtle, where the commenter actually tries to leave a relevant message on the blog. 

To summarize: Spammers hope to send their comment spam to your blog so that the link they have embedded in their comment will create a “backlink” ( linking your site to theirs) thus making their site seem more popular and relevant to search engine robots. This can result in their site getting a better page ranking. You may not wish to be the unwitting host parasitized by spammers. Spammers do not ask your permission to use your site for their sole advantage.

     Spam was not much of a problem for me (until recently,) but as anyone with an established website can tell you, as soon as your traffic increases , so to, unfortunately, does the volume of comment spam.   When you set up your blog, you can expect the same and that is why it is a very good idea to make sure you have installed a spam filter on your web site.

 Take care and hope to have you visit again soon.—wil