Web 2.0 brought us basically a better way to interact with web resources and made it possible for millions of people to connect with each other via social networking sites and through wikis, blogs and folksonomies, podcasts, RSS... In fact the key idea of web 2.0 is collaboration between users. Web 2.0 is not a new platform, but rather a "business revolution" as Tim Berners Lee said. It's changed the way we interact, the way we work, the way we live. Businesses have started sharing their information, products and services through the web 2.0 medium and we are starting to interact with these businesses as customers in a different way.
What is web 3.0 then? Well to be honest we don't really have a definition as such yet. It's early days. Web 3.0 is about moving forwards with the Internet, about improving it, and yes, it's about data, and what we do with it...and what the machine does with it. The important part of web 3.0 for me is the machine's ability to manipulate data rather than just displaying it. I work in natural language generation and machine understanding and the overall path seems to be towards allowing machines to understand web content and create their own. You could ask it to buy stuff for you on amazon, or book you a flight or a hotel. An early example of this in my opinion is the work being done in music recommendation.
Tim Berners-Lee's definition is "an overlay of scalable vector graphics - everything rippling and folding and looking misty - on Web 2.0 and access to a semantic Web integrated across a huge space of data, you'll have access to an unbelievable data resource."
My definition is , web 3.0: artificial intelligence to allow for a web with reasoning capabilities built in.
The technologies used for web 2.0 are (amongst some) REST/XML, CSS, Microformats, Ajax, RSS, folksonomies. The technologies used for web 3.0 are description logics and intelligent agents. I'm working in this area and I'm using web 2.0 technologies and getting intelligent agents to interact with them for example.
You might also enjoy SCIgen, a system that generates computer science research papers all on it's own. Sadly it's just for fun so I won't be able to use it to avoid writing papers :(
What does it mean for SEO? Well i don't think it's going to die out any time soon, but I think the way it happens will change. Already with web 2.0 we're using social media, most companies have a blog, podcasts even, most provide newsfeeds,...we've embraced web 2.0 in our seo efforts. Web 3.0 (when we finally have the technology to make it happen) will be no different. It'll become more important than ever to create well focused sites that a machine can breakdown and draw information from for use in another medium. We'll also need to cater for the user as well as possible in order to encourage them to choose our site information over another's. I think that this is going to have more to do with the information in the site rather than the site itself. In my research for example, the user doesn't even go to the site, just interacts with the information.
An interesting thing to note is that my system works with the user and also makes up it's own mind. There are many systems in research that are using this method. Do we finally have a partnership between man and machine? The user fine tunes and the machine learns and creates...The machine has a long way to go yet, and so does the user.
Some resources for you: a primer on the "semantic web", Eric Schmidt defines web 3.0,