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.

Advanced VALUE Mask and Snorkel Set £6.99	TBF ADULT set A Mask and Snorkel Set - PINK £9.99
TBF ADULT set A Mask and Snorkel Set - RED £9.99	TBF ADULT set A Mask and Snorkel Set - BLUE £9.99
TBF ADULT set A Mask and Snorkel Set - YELLOW £9.99	Typhoon TM2 mask SET- BLUE Suits Medium to Large face £20.99
Typhoon NEW TM1 and TS3 Set - Blue Suits Small to Medium face £29.99	Typhoon NEW TM1 Mask Set - Silver Suits Small to Medium face £29.99
TBF set 3 Mask and Snorkel Set - BLUE Suits Medium to Large face Dry Top upgrade option £19.99	TBF set 3 Mask and Snorkel Set - BLACK Suits Medium to Large face £19.99
TBF set 2 Mask and Snorkel Set - BLUE Suits Medium to XL face Dry Top upgrade option £19.99	TBF set 2 Mask and Snorkel Set - BLACK Suits Medium to XL face £19.99
TBF Mask and Snorkel Set - RED SET 1 OR 3 - SILICON £19.99	TBF Mask and Snorkel Set - AQUA SET 1 OR 3 - SILICON £19.99
TBF Mask and Snorkel Set - PINK SET 1, 3 or 4 - SILICON £19.99	TBF set 1 Mask and Snorkel Set - BLUE Suits Smaller adult face Dry Top snorkel option £19.99
TBF set 1 Mask and Snorkel Set - BLACK Suits Smaller adult face Dry Top snorkel option £19.99	TYPHOON STEALTH TM2 BLACK mask and snorkel SET Suits Medium to Large face £20.99
Typhoon LADIES PINK TM3 mask and TS3 snorkel Set Suits Ladies face - Dry top option £29.99	Typhoon LADIES PINK TM2 mask and Snorkel Set Suits Ladies face £20.99
Typhoon LADIES Titanium Silver Set Suits Ladies face £29.99	TYPHOON Ladies - TM3 SET - Yellow Suits Ladies face £29.99
BEAVER ANTI-GLARE Mask + TBF Dry Top Snorkel - SET £36.99	TYPHOON - TM4 mask and Snorkel - SILVER Designed for kids or ladies smaller face £19.99
Typhoon CLASSIC and TS3 Set - Yellow Suits Medium to XL face £29.99	TYPHOON - TM4 mask and Snorkel - Blue Designed for kids or ladies smaller face £19.99
TYPHOON - TM4 PINK mask and Snorkel Designed for kids or ladies smaller face £19.99	Seac Sub U-Fit COMBAT Mask + Snorkel set £46.99
Seac Sub U-Fit BLACK Mask + Snorkel set £46.99	Sphera SET in PINK - By Aqua Lung Sport £29.99
TYPHOON - TM4 mask and Snorkel - YELLOW Designed for kids or ladies smaller face £19.99	Anti-Fog liquid *****MUST HAVE***** £4.99
Snorkel Keeper / clip £1.99	SEAC SUB Snorkelling snorkeling Vest £22.99
BEAVER Snorkel Keeper / clip £3.99	Mmmmm ... Nice Beaver ... replacemant Mask Strap cover £6.99
Sphera SET in BLUE - By Aqua Lung Sport £29.99	Sphera SET in BLACK/SILVER- By Aqua Lung Sport £29.99
BEAVER ANTI-GLARE Mask ++ OMEGA Dry Top Snorkel - SET £31.99	Beaver Mask Strap cover - Choice of colour £4.99