Masks are made from 3 types of material:
PVC - a cheaper product and harder. Therefore not so comfortable on the face.
Silitex - A slightly better form of PVC
Silicon - VERY comfortable on the face and can be worn for a longer period. Also, has a much better seal against the water getting in. The silicon masks are generally made to a higher quality
My preference is for a silicon mask. I would not dive or snorkel with anything else.
Masks are designed to cover the nose to stop you sucking in water when using a snorkel. The nose area can be pinched so that you can equalize when diving down or using scuba equipment.