It actually refers to a set of sites that is outside of your own, but allows you to interact with others in a meaningful way. Social media sites can be thought of as a type of Web 2.0 property, as well as certain bookmarking sites like Digg, Stumbleupon and Squidoo.
These sites allow you to post content such as articles, videos and blog posts so that users of the community can vote on them, forward them to friends and interact with you. The number and names of Web 2.0 properties changes frequently. The way that they work within Google also changes since Google is known for altering the way that its algorithm works from time to time.
Using Web 2.0 is another great way to vary the types of links that you are sending to your site. Google likes to see variety when it comes to links. In other words, you don’t want ALL of your links to come from one place as that would look unnatural.
So, make sure that Web 2.0 has its place in your online marketing plan!