Web 2.0
 

Web 2.0 websites include the following pieces of functionality.  The acronym SLATES as used by Andrew McAfee:

Search, Links, Authoring, Tags, Extensions and Signals.

Search

Give users the ability to search your site.

Links

Hyperlinks connecting related content within your site and the internet.

Authoring

Allow users of your site to add/change content, blogs etc.

Tags

Allow the addition of identifiers to your site thereby assisting searching.

Extensions

Extending your sites functionality through add-ons, such as PDF readers, Flash players and Silverlight etc.

Signals

The use of technology, such as RSS to notify users.

There is additional information here on Web 2.0