At this point in the 21st century, everyone is familiar with what a blog is. In SharePoint, a blog is a site that is characterized by entries of content being listed from newest at the top of the page to oldest at the bottom. Content is determined by the author and can contain metadata such as the author, date, and keywords or tags to designate the categories in which the content belongs.
A blog site can either be the root site or a site contained within another site. This is consistent with all site types in SharePoint. The reason this is pointed out though, is that when creating a new SharePoint engagement, it is important to truly think of the overall goal of the website as a whole. If the website will essentially be a blog with other content thrown in from time to time, then it makes sense for the root site to be of the type blog. Otherwise, it makes sense to make the root site something else like the team site or a blank site and then add a blog...