Shape - Cross Country Skis typically come in two different shapes.
Sidecut Cross Country Skis are wider in the tips and tails and are most commonly found in Backcountry and Groomed/Ungroomed skis. The wider tips will keep you on top of the snow, and break trail better. Skis with sidecut are easier to turn, especially on downhill runs.
Reverse Sidecut Cross Country skis are wider at the waist than they are in the tips and tails. Reverse Sidecut skis are found in Skating or Groomed cross country skis. Having a ski that is wider at the waist gives you more stability and balance on a groomed track. When the widest point of the ski is directly underfoot it increases the amount of glide you get with each push.