As their name implies, nordic backcountry boots are designed for cross-country skiing in the backcountry. As a result, they feature an insulating layer that provides extra warmth, along with a gaiter that extends over the cuff to keep snow out.
Because most cross-country skiing bindings are not compatible with each other, it is important to find boots that both fit comfortably and are compatible with the bindings on the skis you will be using.
Hikers looking to cross country ski can do so with hiking boots that are equipped with special binding technology but they're not for hardcore nordic enthusiasts, this is more for a beginner or recreational skier who wants to try something new and different.
Cross country ski boots are designed to attach only the toe of your boot to the ski, while downhill skiers need additional support because they have to corner or lean on their skis. The rigidity and support provided by a downhill boot differs from cross-country skiing boots as they are designed with more flex to allow for more natural movement when hitting bumps on the trail.
A good cross-country ski boot should allow your toes to move freely and be flexible. While it's better to size down than to size up, most boots come with a pocket so you can insert additional foam or other materials if you need additional warmth or comfort.