Vellus hairs are the thin whispy hairs that we have all over our body, and don't really have any pigmentation (color). Take a close look at your forehead (not your hairline - more between your hairline and eyebrows) and you will see you have thousands of tiny little white hairs that have no cosmetic effect.
The way male pattern baldness works is that terminal hairs (the big, colored hairs you have on your head and some other places on your body) degenerate into these smaller vellus hairs. If you look at the scalp of a balding man, you'll probably see thousands of little vellus hairs, unless they are well into the process and then even the vellus hairs can die out.
Adenogen and minoxidil are growth stimulants, and in theory they work by stimulating these tiny little hairs to grow into the full, terminal hairs that actually give a cosmetic effect.
As for Adenogen, we probably won't know how well it actually works for months. The product was just released to the public yesterday, and the results will surely take a long time to be seen.
