Shortened version of 'weblog'. Originally the weblog was a form of online diary where people listed the things they did during the day. These were usually so boring they quickly expanded into a means of voicing opinions otherwise left unheard. Well they are *still* unheard, but at least they are 'out there' for anyone to dig up. Currently the 'blog' has expanded in use almost to an art form and there are now millions of blog sites where people who feel otherwise disenfranchised can voice their opinions.
The tide seems to be beginning to turn however as more and more people realise that essentially writing to and for oneself is a less satisfactory a means of communication that they'd thought.