1. A method of categorizing a web site on a network, comprising: using a computer to perform steps comprising: receiving a web site browsing stream describing temporal proximities of web sites browsed by a client, wherein a temporal proximity indicates a time interval between visits to browsed web sites; identifying a category of a first web site in the web site browsing stream; determining a temporal proximity of a second web site relative to the first web site in the web site browsing stream; determining a probability that the second web site is of a same category as the first web site in the web site responsive at least in part to the temporal proximity of the second web site relative to the first web site and a degree of content-temporal locality for the category of the first web site; and storing the determined category probability for the second web site.