From my understanding (very limited):
Web 1.0 is mostly read-only web.
Web 2.0 came along with read-write web, that allows users to add content (blog, noticeboard, fluther)
Web 3.0 will be read-write-execute web; That is, web that allows the user to create and use tools.
(however web 3.0 is still being discussed and it can really be defined as any number of things)
Another view:
Web 1.0 is like a book, you can read it, can't really change it.
Web 2.0 is like an operating system, with a lot of seperate little applications.
Web 3.0 will be like one application with a lot of seperate features.
However I believe web 3.0 is still a long way off, it's still difficult for people to provide an adequate definition of web 2.0.
(if you want to talk graphically, web 1.0 is flat graphics, web 2.0 is shiny reflective simple graphics, speculation says web 3.0 will be 3d graphical interfaces)
So there are many answers to this question, all of which contradict each other, and all of which are valid.