Web 2.0 is the second generation of the internet.
The first generation of the internet (Web 1.0) was a "read only" web for the majority of internet users. If users did not possess the right tools or technologies to create a website they could only read what other people have published.
The second generation of the internet (Web 2.0) is a "read-write" web for the majority of internet users. Users now have access to a wide variety of online tools that enable them to create and publish their own materials on the net.
Web 2.0 is all about
Source: Web 2.0 Survival Guide
(What is Web 2.0? was created at Tyndale University. Permission to use was given by Mark Mueller.)