Ski and snowboard mittens are generally made from leather or a combination of textiles with leather or synthetic leather palms. Leather may require longer break-in and occasional treatment, but it's very durable and molds to your hand over time. Textile mittens are lightweight, flexible, and dry quickly. Mittens with synthetic leather palms are less expensive.
Insulation traps heat to keep your hands warmer longer. Because of their design, mittens for snowsports often feature more insulation than gloves. Synthetic insulation is usually used because it performs better in wet weather, but you can find puffy mitts with down insulation as well, particularly for mountaineering or extra-cold weather.
During the winter, a wet mitten poses as much danger as cold temperatures. Look for a mitten with a waterproof breathable shell or internal membrane like Gore-Tex, eVent, Outdry, or a brand's proprietary technology, and compare waterproof ratings. At the very least, you'll need a mitten with a water-resistant coating.
The cuffs for mittens fall into two categories: gauntlet or undercuff. A gauntlet-style mitt slips over the sleeve of your jacket and cinches to keep the elements out. This design can feel bulky to some, so if you want a sleeker fit look for an undercuff style with a slim profile that slips under your jacket cuff.