Based on the concentration /stregth of fragrance, perfumes can be classified into Perfume, Eau de Parfum, Eau de Toilette, and Eau de Cologne. The differences are simply a matter of the amount or concentration of oils in the fragrance. These oils are called "juice." The highest concentration of "juice" is in perfume (or parfum). Next would be Eau de Parfum, then Eau de Toilette, and finally Eau de Cologne.
More specifically, to how much high grade alcohol and/or water has been added to the fragrance oils justify the classification. Eau de cologne (generally the mildest form you can buy) has 2 - 5% perfume oil dissolved in alcohol. Any mixture with a lower proportion of oil to alcohol is an eau (water).