Wikipedia defines blogs as:
A Blog (a contraction of the term “Web log”) is a Web site, usually maintained by an individual with regular entries of commentary, descriptions of events, or other material such as graphics or video. Entries are commonly displayed in reverse-chronological order.
The Blogosphere is the collective community of all blogs. Since all blogs are on the Internet by definition, they may be seen as interconnected and socially networked. Discussions “in the Blogosphere” have been used by the media as a gauge of public opinion on various issues.
Taken from Technorati’s 2008 State of the Blogosphere.