A blog oftentimes relays the viewpoint of an individual. A traditional web site can be home to opinions as well, but you can think of blogs as an online journal. A blog can create a community in which a discussion is posed and other followers respond; creating an on-going dialog between the manager of the blog and readers. This same effect can be created using a traditional message board thread as well. Companies and individuals that deal in the news business have really adopted the blog as a way to translate their stories and opinions. In addition, blogs have blurred the line between journalism and professional journalism.
Blogs tend to not have as much integrity as a traditional website. Blogs can point consumers in the direction toward commerce, but due to their nature, are not usually the center for commerce.