Perhaps the only thing on which everyone can agree about Web 2.0 is that it has become a potent buzzword. It provokes enthusiasm and cynicism in roughly equal measures, but as a label for an idea whose time has come, no one can seriously doubt its influence.

So what does it mean? Web 2.0 began as a conference,¹ first hosted in October 2004 by O'Reilly Media and CMP Media. Following the boom-bust cycle that ended in the dot-com crash of 2001, the organisers wanted to refocus attention on individual web success stories and the growing influence of the web as a whole. True, during the late 1990s hype and expectations had run ahead of reality, but that did not mean that the reality was not epochal and world-changing. By the following year, Tim O'Reilly, founder of the eponymous firm and principal articulator of the Web 2.0 vision, had laid down in a seminal essay² a set of observations about approaches that work particularly well in the online world. These included:

• "The web as a platform"
• The Long Tail (e.g., Amazon)
• Trust systems and emergent data (e.g., eBay)
• AJAX (e.g., Google Maps)
• Tagging (e.g., del.icio.us)
• Peer-to-peer technologies (e.g., Skype)
• Open APIs and 'mashups' (e.g., Flickr)
• "Data as the new 'Intel Inside'" (e.g., cartographical data from MapQuest)
• Software as a service (e.g., Salesforce.com)
• Architectures of participation (e.g., Wikipedia)

The sheer range and variety of these concepts led some to criticize the idea of Web 2.0 as too ill-defined to be useful. Others have pointed out (correctly) that some of these principles are not new but date back to the beginning of the web itself, even if they have only now reached the mainstream. But it is precisely in raising awareness of these concepts that the Web 2.0 meme has delivered most value. Now, those of us without the genius of Jeff Bezos or Larry Page can begin to glimpse what the web truly has to offer and, notwithstanding the overblown hype of the late 1990s, how it really is changing the world before our eyes.

Initially the first item in the list above – the web as platform – seemed to have primacy among the loose collection of ideas that constituted Web 2.0 (see, for example, Figure 1 in ²). The most important thing seemed to be that talent and enthusiasm in software development was migrating from traditional operating system platforms to the web. New applications were agnostic with respect to Unix versus Macintosh versus Windows and were instead designed to operate using web protocols (specifically, HTTP and HTML) regardless of the precise underlying software running on the server or client machines.

However, this view taken on its own overlooks one very important reason why that migration has happened: the web is more powerful than the platforms that preceded it because it is an open network and lends itself particularly well to applications that enable collaboration and communication. With his usual eye for pithy phrasing, Tim O'Reilly described this aspect using the terms "architecture of participation"³ and "harnessing collective intelligence."² He pointed out that the most successful web applications use the network on which they are built to produce their own network effects, sometimes creating apparently unstoppable momentum. This is how a whole new economy can arise in the form of eBay, why tiny craigslist and Wikipedia can take on the might of mainstream media and reference publishing, and why Google can produce the best search results by surreptitiously recruiting every creator of a web link to its cause. In time, this participative aspect came to the fore, and these days "Web 2.0" is often seen as synonymous with websites that do not merely serve users but also involve them, thus enabling them to achieve that most desirable of business goals: a service that gets better for everyone the more people use it.

This brief survey will use a relatively broad definition of Web 2.0. So, while it will deal mainly with participative services and network effects, it will also cover certain other aspects of the original Web 2.0 vision that have particular relevance in science, including mashups and tagging.