If you’re out of the loop, blog is short for Web log; it is like a dated online journal entry that others can comment on. Blogs are a great medium for spreading ideas and having up-to-date discussion on current events. There are free tools available for starting a blog, so blogging is available to anyone who has a domain.
The beauty of blogs is they promote the free exchange of ideas.

Blogs and SEO

Search engines like Google (and Yahoo! to a much lesser extent) are interested in keeping up with the latest and greatest, so their results pages contain fresh information. Blogs help you get new content onto your site, and it gets search engine spiders to your pages more frequently. In addition, blogs are often naturally laced with keyphrases and contain information that will bring unique visitors to a site.

Blog Spam

Sites that allow commenting with a signature (a signature is a link that a commenter leaves under their message) are often spammed. Users leave unrelated comments and post a link to their site with keyword stuffed linking text in order to build link popularity. Comments should be subject to at least a small amount of editorial review.
Business sites that know the benefits of blogging will often come up with ridiculous blogs related to their products (blogs about gum, hair cream, whatever). These blogs can barely be counted as spam because they are never successful. They fall under the category of spam because of the user’s intent: creating a blog only with search engines in mind.