Not sure about boys stuff but with girls parfum (cologne) is like a pure perfume form, you'd just dab a couple of drops on, eau de toilette is a watered down version that usually comes in a spray that you would put more of on, not sure about aftershave, think it's usually the same as eau de toilette...
The amounts of fragrance to alcohol is the difference.
Aftershave is usually 5 per cent
Cologne usually 10 per cent and EDT is the strongest with more than 10 per cent.