Can anyone tell the difference between several cosmetics brands?

I have recently been noticing the new brands of cosmetic arriving in the market. Whenever I pick a magazine or a newspaper there is a add of new cosmetic product or about the new brands that are launched in the market. These adds have made me little skeptical as all of them promises the same thing. I want to know is there any differences in all these cosmetic products or all of them are just same? As I have normal skin so what kind of cosmetic product should I go for?