If all the buzzwords and acronyms (Web 2, AJAX, RSS, XHTML, XSLT, tags, blogging, podcasting) are hurting your head, no fear — none other than Tim O’Reilly himself has written a nice essay titled “[[http://www.oreillynet.com/lpt/a/6228|What is Web 2.0?]].
While not the most comprehensive and the most insightful piece, it is nonetheless a very good overview. If it seems too long, just go through the 8 bullets down at the bottom for the meat of the matter.