Amateur videos, blogs and podcasts are categorized not only by their creators, but also by anyone who cares to save the content and tag it. This tagging, both by the original content creators and by those who use tagging systems such as del., is both a search and a social function, according to a new report by the Pew Internet & American Life Project.

"Millions and millions of people are saying, in public, what they think pages and images are about. That‘s crucial information that we can use to pull together new ideas and information across the endless sea we‘ve created for ourselves," said David Weinberger, the author of the Pew report.

And who are these millions and millions of people? As with most new things online, the early adopters tend to be young, educated broadband users with high incomes.

Perhaps nowhere is the importance of commenting on user-generated content as great as it is with video. User-generated Super Bowl ads are only the most visible example of how far such video can go; online, it is the push of amateur evangelists that propels video into meme territory. Appending "funny," "sexy" and other tags to videos can bring additional viewers via tag searches.

The trend toward user-generated video is expected to rise: 55% of online video content will be user-generated by 2010, up from 47% in 2006, according to a January 2007 report by Screen Digest.

The viral marketing possibilities for user-generated video are not lost on marketers. According to the American Advertising Federation, 19% of US advertisers had advertised on a user-generated content site as of June 2006. And an additional 14% planned such advertising in the coming year.

David Hallerman, author of eMarketer‘s recent Internet Video: Advertising Experiments and Exploding Content report, notes that consumer-generated content has been the platform for ad-supported video before.

When it comes to spreading the word about online video, however, not all tagging is born of good intent. Many user-uploaded videos on YouTube contain tags for things that are not in the video at all ("Nintendo Wii" is one benign example), in an effort to promote less-than-stellar content or spam.