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.

Bahia Mask and Snorkel Set For Teenagers and Adults £13.99	Bora - Mask and Snorkel Set For Teenagers and Adults £15.99
Typhoon TM1 and TS3 Set - Blue Suits Medium to XLarge face £29.99	Typhoon TM1 Mask Set - Silver Suits Medium to XLarge face £29.99
Typhoon TM1 and TS3 Set - Yellow Suits Medium to XLarge face £29.99	TYPHOON TM2 BLACK mask and Dry snorkel SET Suits Medium to Large face £23.99
Typhoon LADIES PINK TM3 mask and TS3 snorkel Set Suits Ladies face £29.99	Captain Seebreeze set WITH DVD - Mask and snorkel Set £19.99
DEVIL Mask and Snorkel set - SILICON £19.99	Typhoon LADIES PINK TM2 mask and TS3 snorkel Set Suits Ladies face £29.99
Typhoon LADIES Titanium Silver Set Suits Ladies face £29.99	Typhoon TM2 mask SET- BLUE Suits Medium to Large face £27.99
TYPHOON Ladies - TM3 SET - Yellow Suits Ladies face £29.99	BODYGLOVE Down Vu - Mask and Snorkel Set Suits Medium to Large face £29.99
MARE mask and DRY snorkel £29.99	TYPHOON - TM4 mask and Snorkel - SILVER (TITANIUM) Designed for kids or ladies smaller face £19.99
Advanced VALUE Mask and Snorkel Set £9.99	Aqua Sphere - Mask Strap Cover £4.99
BODYGLOVE - Mask Strap Cover WITH new mask strap £6.99	Replacement Mask Strap - Silicon £2.99
Sphera Mask and Snorkel SET by Aquasphere - All Black £23.99	TYPHOON - TM2 mask and TS3 Snorkel - YELLOW Suits Medium to Large face £28.99
TYPHOON - TM4 mask and Snorkel - YELLOW Designed for kids or ladies smaller face £19.99	Anti-Fog liquid *****MUST HAVE***** £4.99
BODYGLOVE - Mask Strap Cover WITH new mask strap £6.99	CooLock - Waterproof Bag £4.99
EASYPIX W311 - Digital Underwater Camera £69.99	Eco - PVC - Waterproof Bag £2.99
Java Tropic Mask and Snorkel Set Teenage or Smaller face set £9.99	Replacement Mask Strap - Silicon £2.99
Snorkelling Vest - Typhoon £23.49