Yet Another Hierarchical Officious/Obstreperous/Odiferous/Organized Oracle.
(Or a member of a race of brutes in Swift's Gulliver's Travels who have the form and all the vices of man, or an uncouth or rowdy person).
Probably the biggest hierarchical index of the World-Wide Web. Originally at Stanford University, Yahoo moved to its own site in April 1995. It allows you to move up and down the heirarchy, to search it and to suggest additions. It also features "What's New", "What's Popular", "What's Cool" and a random link.