Mailing lists are very good for certain kinds of communication, but their lack of organization can be a serious drawback. Early in the history of the Internet, a more systematic system was developed called usenet. This system organized the major topics of conversation into a number of large domains. Each domain was then split into subdomains and subsubdomains. (There's that darn tree structure again!) Some of the main domains were
The idea was a user could find exactly what topic she was looking for by navigating the hierarchical structure.
The ideas were expanded when the Internet grew, but the basic structure remains the same.