I personally don't believe there is any difference when it comes to moisturizers. Many of these companies will develop products for individual areas only so their customers end up buying 4 products as opposed to one. I really think it's all just a marketing scheme. One exception would be body moisturizers which are not always suitable for the face. Same goes for eye creams. Many eye creams are suited for the sensitive areas around the eyes and aren't really designed to be used on the entire face. But I see no reason why you couldn't use a facial moisturizer on your neck.