“A blog is a user-generated website where entries are made in journal style and displayed in a reverse chronological order.” Wikipedia
“A weblog is, literally, a "log" of the web - a diary-style site, in which the author (a "blogger") links to other web pages he or she finds interesting using entries posted in reverse chronological order.” from Guardian Unlimited
“I love weblogs because they're yet another way for people to express themselves online.” What the hell is a weblog a personal opinion by Derek M. Powazek
“Weblogs are just too varied, too idiosyncratic, to fit into an existing box. Industry analysts might call this disruptive technology because weblogs have changed personal publishing so profoundly that the old rules no longer apply. We are at the beginning of a new age of online publishing - and I predict that this generation of online pamphleteers is just the first wave.” Rebecca Blood Guardian Unlimited
And here's a video that sums up what blogs are in a nutshell.