The biggest difference between three-season and winter hiking is the walking surface. Snow and ice require traction aids such as snowshoes, microspikes, or crampons. Snowshoes provide flotation so you can walk on top of snow and not sink with each step, an exhausting experience known as post-holing. Microspikes and crampons provide extra traction for walking on ice. They take up where snowshoes leave off and many winter hikers carry both types of traction on day hikes or overnight trips into the winter backcountry.
If you're relatively new to the winter hiking scene, you should probably buy a pair of snowshoes before investing in any other type of winter traction. In addition to providing flotation, modern snowshoes have metal teeth on their undersides, helping you to hike up hills on fresh snow or packed trails. Manufacturers such as Tubbs, MSR, Atlas, or Crescent Moon, offer a wide range of snowshoes that are designed for everything from recreational snowshoeing to full blown mountaineering. Read How to Choose Snowshoes for an explanation of the different types.
Many winter hikers also use microspikes for hikes that require scrambling over ice covered rock. Microspikes are a lightweight traction device with short cleats, attached to an elastic band that you slide over your shoes. Kahtoola Microspikes are a popular choice that are very durable and compatible with all types of winter footwear.
While microspikes are great for walking on icy trails, they don't provide sufficient traction for walking on steep ice covered slopes. These conditions require crampons, a more aggressive traction device that has longer and sharper spikes and can penetrate into ice and give you a better foothold. You can choose among different types of crampons depending on your desired activity and the type of footwear you use. The general rule of thumb is that stiffer soled boots are required for hiking or climbing steeper, more challenging terrain.
If you want to hike up steep hills and mountains with standard hiking boots, there are a number of different options available. You can buy strap-on crampons like the steel Black Diamond Contact Crampon or the lighter, but less durable aluminum CAMP XLC 490 Hyperlight. Strap-on crampons attach to your boots using nylon or plastic straps that loop around your toes and ankle. They can be used interchangeably with many different kinds of boots, making them an economical choice if you switch to heavier boots as the weather gets colder.
If you plan to do general mountaineering or ice climbing with mountaineering boots, you want a step-in crampon like the Black Diamond Cyborg Pro or the Grivel G14. Step-in crampons lock onto heel and toe welts on the rigid soles of mountaineering boots, providing a more secure connection than a strap-on binding. Made out of steel for durability, these crampons are very heavy and often weigh more than one pound each. They can be dangerous to use without proper training, so don't use them unsupervised until you've learned how to use them safely.
Hybrid Traction Devices
If you want to use softer shoes like trail runners in winter for trail running, racing, or simply because they're lighter than wearing heavy boots, you should consider some of the newer hybrid traction devices that are designed for less traditional winter footwear.
For example, the Hillsound Trail Crampon Pro has two cm teeth that are midway between the one cm cleats found on Microspikes and the three cm teeth found on many step-in or strap-on crampons. They can be used with any style of boot and use a snowboard style, ratchet binding that is easier to put on and take off than a strap-on binding. The Kahtoola KTS Crampons, available in both steel and aluminum, also have two cm teeth. They are designed for use with trail runners, including an extremely flexible leaf spring connecting the front and rear parts of the crampon.
Additionally, there are instep crampons, titanium hobnails you can screw into trail runners or hiking boots, and even snowshoes with removable crampons than can be worn separately to save the weight of carrying two separate traction aids. The problem of winter flotation and traction has obviously vexed humankind since the Ice Age!