Web 2.0 websites allow users to do more than just retrieve information. Users can own the data on a Web 2.0 site and exercise control over that data. Web 2.0 websites typically include some of the following features and techniques.
1. Search: Finding information through keyword search.
Links: Connects information together into a meaningful information ecosystem using the model of the Web, and provides low-barrier social tools.
2. Authoring: The ability to create and update content leads to the collaborative work of many rather than just a few web authors. In wikis, users may extend, undo and redo each other's work. In blogs, posts and the comments of individuals build up over time.
3. Tags: Categorization of content by users adding "tags" - short, usually one-word descriptions = to facilitate searching, without dependence on pre-made categories. Collections of tags created by many users within a single system may be referred to as "folksonomies" (i.e., folk taxonomies).
4. Extensions: Software that makes the Web an application platform as well as a document server.
5. Signals: The use of syndication technology such as RSS to notify users of content changes.
References: http://en.wikipedia.org/wiki/Web_2.0
