This winter marks my first season of competitive individual skimo racing.
But first, let’s clarify what I’m talking about.
WHAT IS SKIMO? Is it SKIING? Is it CROSS COUNTRY SKIING? Is it BACKCOUNTRY SKIING? What are skins? How do you go uphill? What’s the point of it?
The following explains the basics, through my eyes, the eyes of a runner. A more experienced backcountry skier would explain this more thoroughly, but this should elucidate the most basic of your questions. My beloved runner readers who don’t know what the heck skimo is. Why should you?

Going up! Skins on bottoms of skis, free heels, and flexibly ankle mobility in boots’ uphill mode. 

Skimo: short for ski mountaineering

It is far from cross-country, Nordic, or skate skiing. You can’t ski up or down big mountains doing any of those. Ski mountaineering utilizes fast AT setups. What’s an AT setup? AT stands for alpine touring, which means you tour the mountains on your own human power—no ski lifts or snowmobiles—by using AT-specific skis/boots/bindings/skins. The bindings and toe-mechanism on the boots allow for one’s heels to lift up on the way up a mountain. It’s uphill walking on skis. You attach sticky synthetic ‘skins’ to the bottom of your skis, which use the power of physics and friction to prevent you from sliding backwards (most of the time—icy conditions make for difficult, technical skinning).

Free heels on the uphills allow for creativity. Matt Malone breaking trail in Montezuma. 
Attempting to put on a skin that’s become cold and iced over, meaning the stickiness is gone…My poor Dad on last year’s Grand Traverse. 
Once you’ve summited your objective, time to ski downhill. Boots are snapped into ‘downhill mode,’ which stiffens the boot shaft into a standard resort-type of boot, removing the ankle flexibility that you have in ‘uphill mode.’ Skins are removed by stripping them off the bottom of skis—skins have a sticky glue backing that sticks to skis, but rips off without leaving residue, time after time—and boots are latched into bindings in downhill mode, securing your heels to the ski, like a standard alpine resort ski setup. You shove your skins into your shirt, to keep them from icing over in frigid conditions, creating an attractive-belly-protrusion look (specific to skimoers who don’t take the time to put skis in a backpack).
Downhill skiing with a light skimo setup means less shredding in powder. I’m skiing timid here because my skis are practically weightless! 
Coach Joe Howdyshell and Henry model their belly pop of skins. 
Unlike backcountry skiing, which utilizes all of the same foundations as skimo, skimo implies the fastest version of alpine touring. The setup—boots, skis, bindings, skins, poles, and avalanche gear (probe, beacon, shovel)—is judged by its weight: the lighter the better. Backcountry skiers condemn the disgustingly light gear of skimoers because it detracts from the ability to shred downhills. A heavy, wide powder ski makes for a smooth, buttery and delicious downhill, but a racing skimo ski—a toothpick compared to a powder ski—is difficult to manage and isn’t necessarily delicious. It’s made for fast uphills. Period.
Skimo setup on left; wider, powder skis on right. Both in uphill mode with skins on bottom of skis and free heels. My brother Scottie was able to move a lot faster uphill than me since he was using lighter gear. 
An important nuance to all of this is the avalanche gear: a beacon, which is a standardized transmitter that connects to other beacons. If there’s an avalanche, the people not stuck in snow become searchers, furiously scanning the snow with their beacons, looking for the buried person(s) via his or her own beacon’s signal. A probe is a long collapsible tool to shove into the snow, to look for buried people. A shovel is used to dig out buried people. When anyone is outside of the cushy confines of a ski resort, which controls for avalanche danger, avalanche tools are imperative. As is basic avalanche knowledge learned in an avalanche course, AIARE 1. More to follow on my first competitive skimo race in Idaho last weekend. Even though I had fast skimo gear, I wasn’t moving like the rest of my competitors…
The lighter the gear, some may say, the less comfortable the gear. Earthraging nonetheless! 

What the heck is skimo?