Why tags? Tags are an alternative to hierarchical categorization. And alternatives are necessary because hierarchical categorization systems break down as the volume of content grows exponentially. For example, Yahoo! (or at least the original Yahoo!) used pre-defined categories to organize their database of websites. But their categories couldn't keep up with speed at which sites were being added to the web. Consider how many levels had to be navigated before getting to a helpful list of sites. Digging down three, four or even five levels to get to a helpful list of sites was not uncommon. Such search results may have a hundred or so sites on them. But what happens when that list of sites grew to three hundred sites and then three thousand? A list of thousands of sites is too much to digest, so new layers of categorization become necessary. The more levels the more refined the categories become and the greater specialization needed by directory editors. As the vast range of categories, sub-categories, sub-sub, and sub-sub-sub-sub categories increase, the system itself becomes less and less helpful.

Formal categorization systems like Yahoo! uses are referred to as taxonomies. Informal system of categorization, like the way individuals assign their own words (tags) to describe their bookmarks, are referred to as a folksonomies.

So the first difference between a browser's favorites menu and del.icio.us is that del.icio.us allows users to assign personal tags to bookmarks rather than assigning sites to hierarchical categories.   next >