With the second annual Web 2.0 Conference convening in San Francisco on Wednesday, it’s little wonder that the blog world is full of discussion on the topic. Still, it’s a bit of a challenge to get a handle on just what is at the heart of the movement. Among those who lay claim to the Web 2.0 mantle, there are as many definitions as there are definers, but that’s in the spirit of the “radical decentralization” and “architecture of participation” that distinguish the movement.

Advocates are generally in agreement when pointing to examples of Web 2.0 success stories: the active community that’s grown up around photo-upload site Flickr; the ingenious and addictive bookmarking database at del.icio.us; the real-time delivery of video and other large files across BitTorrent’s decentralized network in which every client is a server; the breadth and utility of the user-written Wikipedia project; and, of course, the phenomenal growth of blogs and syndicated feeds.

What distinguishes all these efforts is the fact that they have adopted the web as a platform, delivering services across networks and devices rather than distributing software artifacts. They are all free and, as such, can afford to be in perpetual beta release. Users happily provide testing, feedback, and suggestions. Some even take advantage of the open source code beneath the surface, contributing valuable variations and enhancements.

By design and default, individual users evangelize and populate the databases. The more people use Web 2.0 services, the more robust and useful they become, and all at very little additional cost. Because much of the contributed content is niche, users are rewarded with an unprecedented array of choices.

Indeed, a whole new argot is growing up around the movement. Folksonomy, a portmanteau for folks and taxonomy, describes the popular use of tagging or freely chosen keywords to categorize content from the bottom-up rather than the top-down hierarchies employed by traditional systems. The Long Tail, with its L-shaped distribution curve, illustrates the cost-effective access to niche content allowed by Web 2.0 mechanisms. Remixing is stolen from the pop culture world and is used to describe hybrids like the many homegrown Google Map applications.

Whether these grand experiments in “radical trust” and the “wisdom of crowds” survive and evolve viable business plans is yet to be seen, but the next few days should provide us with some intriguing prognostications from those on the frontlines.