Blogosphere refers to the global blogging community. Term blogosphere means a collection of all the blogs found in the Internet.
The reason this term is used more and more often these days is simply because many blogs get involved in linking to each other, posting comments on each other blogs and do other forms of activity to get interconnected. This creates a sense of having a unified blogging space – blogoshpere – and every blog belongs to it.
Ultimately, it really is up to you as a blogger to decide how involved your blog is going to be with the rest of blogosphere, but it is an undeniable fact that it's much easier to grow traffic, gain readership and generally become visible by taking active participation in blogging community and following general blogosphere trends.
There's quite a number of sites which track blogosphere activity – ranging from simple aggregators to full-scale search engines with lots of cool features to help you stay tuned and get all the recent news from all the blogs of your interest. For the moment, I'd like to recommend you only one: Technorati.