According to Fragrance Net:
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.
Actually, Eau de Toilette and Eau de Cologne are generally interchangeable, particularly in Men's fragrances. After Shave has the least amount of oils. The higher the concentration of "juice" the longer your fragrance will last, and the less you need to apply.