What are the hallmarks of Web 2.0? How is it different from the regular Web?

Web 1.0 was the initial great rush to populate the Web with pages of information connected by hypertext links. In the later years of the period, many of those pages were driven by databases behind the scenes, but the average person couldn't see much difference.
As high-bandwidth, always-on access became more widely available and Internet usage increased, some sites developed ways of utilizing data from that usage. Some of that data was gathered by the systems themselves, such as Amazon.com Inc.'s "also purchased" feature ("Customers who bought this item also bought..."). A great deal was contributed directly by users, generally in the form of short reviews, comments or suggestions. Though relatively few users participated heavily in such collaboration, the ability to collaborate encouraged the development of a remarkable "group wisdom" on such sites.
Over time, collaboration moved to the heart of the Web 2.0 experience. Sites such as those of Craigslist Inc., eBay Inc. and MySpace.com Inc. provide three very different experiences, but all are entirely predicated on user contributions and interactions between those users.
Meanwhile, community-centric sites such as Slashdot and Digg relied on readers to contribute links (usually to news stories) that might be of interest to the community, to describe and comment on the contributions, and to moderate (and thus provide first-line filtering of) other users' comments. This gathering function has become a de facto answer to Web 1.0's information-dispensing model -- rather than consuming the news as presented under a single brand (such as USA Today or The New York Times), those using a Web 2.0 news-gathering model tend to check community-driven aggregation sites for pointers to specific stories on the news sites. A few news-gathering sites dispense with the human element altogether, relying on robots to parse and numerically weighting the importance of news stories as they happen.