If you look at Area 51, you'll notice there are actually 8 categories1:
proposals in subjects like art, humanities, media, literature, and music
proposals in subjects like management, economics, and markets
proposals in subjects like language, religion, society, groups, and politics
proposals in subjects like home, family, education, careers, and self-improvement
proposals in subjects like organizations, occupations, and vocations
proposals in subjects like sports, games, and hobbies
proposals in subjects like the applied, natural, cognitive, formal, and social sciences
proposals in subjects like computing, technology, gadgets, and electronics
It's best to think about these categories as describing the people who are the target audience rather than the subject of the site. Obviously there are affinities (scientific people are often technology people too). However the idea is to group sites in a way that will be broadly of interest to people involved particular social spheres.
When it comes to the organization on Stack Exchange itself, the categories needed to be collapsed in order to fit the graduated sites in the footer. (See below
vvv.) All of the Business and Professional sites are in beta, so those are not included. Of the remaining categories, Technology is far and away the most populous as you might imagine from our origins. The challenge was to fit the other categories in the footer in a pleasing way. Grouping Life with Arts and Culture with Recreation did the trick. The category grouping is extended to stackexchange.com/sites so that when you click the
more (X) link you will get the same set of sites.
As new sites pass from beta to fully graduated, we'll need to add them into the footer. That could very well mean that categories are grouped differently. Certainly if a Business or Professional site graduates, they will need to be added. Another Culture site or two would probably warrant its own column.
- It's entirely coincidental that there are also 8 Usenet hierarchies. There is a fair amount of overlap; Stack Exchange has Business and Professional rather than