In the Web 2.0 definition by Tim O'Reilly there are seven rules.

• The web is the platform. Rapidly replacing the operating systems, which are becoming commodities, the browser with its ecosystem has emerged as the central element of the modern computing platform.
• Harnessing collective intelligence: With blogging, tagging, most-visited lists, ratings, and user-interaction, many web sites get shaped by users' actions, and depend on what users do and write.
• Data is the next Intel inside: Who owns the data? The user or the site owner? Notice, for example, the trend of sites asking for passwords to access your data from other sites. From the technical side, you should also been good at managing the data: Database competence is key.
• End of the software release cycle: Web 2.0 sites are forever in beta, consider users as co-developers, and favor incremental deployment. For example, GMail with its millions of users for a couple of years is still formally in beta. From the technical side, operations becomes a key competence.
• Lightweight programming models: This is a technical element only, based on ideas like lightweight models; loosely coupled systems; syndication, not coordination; design for remixability; and mashups).
• Software above the level of a single device: With such examples as new activities taking the name of a device, like the podcast, YouTube video being offered directly on Google TV, TV viewed on a PC, and micro-blogging from cellphones, there is a significant convergence of devices.
• Rich user experience: Rich Internet Application development, with standard-based techniques like HTML enhanced with CSS and AJAX, or fancy proprietary solutions like Flash from Adobe or Silverlight from Microsoft, have brought the web to a new level.