Web 2.0 properties have become the crest of innovation in SEO, being the latest technique that allows you to get highly-oriented traffic. These properties can be defined as user generated content web pages, which allow users to interact and collaborate with each other.
Think of it as a community, where you can build a reputation due to highly relevant and keyword-focused content. Links coming from these properties are of great value for search engines and allow you to avoid Google Panda & Penguin.
DA is from 20 to 80