Skate skis intended for racing are going to be lighter and have narrower tips than 'sport' or recreational skis, and feature a harder, faster base material that allows them to glide faster than their recreational counterparts.
Skate skis have a camber profile that's longer and flatter than that of XC classic skis, which allows you to push off the edges more efficiently. They also have a stiff flex designed to feel snappy and quick. Racers or those expecting to be on on hard packed, icy tracks will want to look for a somewhat stiffer ski.
If you already have or intend to buy NNN/NIS bindings and boots, look for a ski with an NIS plate mounted to it. If you want to keep your options open, look for one with a plain flat top. On the underside you'll find bases built for speed; since you're using the 'skate' edging motion to go uphill, glide is your only concern. You therefore don't need grip areas or kick wax.