It may sound like another language, and in a way it is: Flickr is a popular photo-sharing service that allows anyone to view most of the more than 50 million member-submitted images it hosts. Tags, meanwhile, are the searchable keywords the individuals can assign to either their own images or to those of nearly anyone else that say something about the information--the defining characteristic of Flickr and a growing number of other online services.

"In Flickr, tags worked because they were fundamentally social," said Stewart Butterfield, Flickr's co-founder. "By agreeing on a tag in advance, users could collectively curate collections of photos in a dead simple way. Now we see people announcing at events, 'The tag for this is baychi05' and stuff like that."

The idea behind tagging may be irresistibly simple, but its ramifications are enormous and complex. For more than a decade, the primary way to categorize and find information on the Internet was through the automated algorithms of search engines, a process at once laborious and highly imprecise. Tagging has quickly gained popularity because it allows human beings to bring intuitive organization to what otherwise would be largely anonymous entries in an endless sea of data.

Also known as "folksonomies," tagging systems are usually created by users themselves, rather than site owners, and make many online services far more accessible and useful than they had ever been before. The practice brings a social context to such resources as blogs, shared bookmarking, photography and even books.

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