A General Classification of Three-Neutrino Models and U_e3

A classification of models with three light neutrinos is given. This classification includes virtually all of the three-neutrino models proposed in the last few years, of which there are approximately one hundred. The essential ideas, attractive features, and characteristic problems of the different classes of model are discussed. The classification is based principally on how the large \nu_{\mu} - \nu_{\tau} mixing is obtained. A general discussion of the mixing parameter U_{e3} is then given, showing what values are to be expected for it in each type of model.Comment: 37 pages, LaTex. Several serious typos correcte