Wikipedia says...
The Blogosphere is the collective community of all blogs is known as the blogosphere. Since all blogs are on the internet by definition, they may be seen as interconnected and socially networked, through blogrolls, comments, linkbacks (refbacks, trackbacks or pingbacks) and backlinks  Discussions "in the blogosphere" are occasionally used by the media as a gauge of public opinion on various issues. Because new, untapped communities of bloggers can emerge in the space of a few years, Internet marketers pay close attention to "trends in the blogosphere".

Several blog search engines are used to search blog contents, such as Bloglines, BlogScope and Technorati. Technorati, which is among the most popular blog search engines, provides current information on both popular searches and tags used to categorize blog postings.