Everybody talks about Web 2.0, Elgg is supposed to be a Web 2.0 application, but few people knows what that really means.
I found a concise definiton of Web 2.0 from the creator of the term (Tim O'Reilly), posted by him in his blog on December, 2006:
"Web 2.0 is the business revolution in the computer industry caused by the move to the internet as platform, and an attempt to understand the rules for success on that new platform. Chief among those rules is this: Build applications that harness network effects to get better the more people use them."
I liked that definition, and added it to the Wikipedia article on Web 2.0.
I also found a marvellous 5 minute video introducing the key ideas behind that term:
Here is the direct link to the video. (nearly 2 million viewers can't be wrong)
I hope this post makes things clearer for those confused about the term. :-)