It's white. Yes, I know what you're thinking, white is easy to create, just overexpose your shot and it gets whiter. Well, you are half right. In that white mass there are subtle variations in brightness and texture. If you overexpose you run the risk of blowing out your highlights and destroying that detail. That detail is crucial because there is so much white.
As always, shoot in RAW mode whenever possible and keep your ISO low.
Your meter is always wrong. Meters read gray, not white. So, you need to adjust your exposure by approximately 1.5 stops. Do this by making an exposure compensation adjustment of +1.5. Or, you can simply choose an aperture or shutter speed adjustment 1.5 stops brighter than the meter suggests.
Use your histogram display, after taking a test shot, to see how close you are. Keep your exposure within 1 stop of perfect. Do not let the exposure hit the edge of the readout. Do not underexpose the snow more than 2 stops, either. Follow these guidelines to avoid clipping and detail loss.
The subject usually isn't white, though. If you expose for a dark subject, you lose the highlights. If you expose for the highlights, you lose the detail in your subject. To deal with this we want an overcast day. Avoid bright sun because it creates to much contrast. We want less contrast in order to more easily find an exposure that compromises for the darker subject and the bright whites.
In the shot below, it was necessary to sacrifice the blue in order to keep detail in the whites. There was also green in the branches to consider. If I had exposed purely for the whites I would have made the blue too light. If I exposed for the rich blue I would have made the whites so dark that they would discolor and lose detail if I tried to brighten them. Luckily the subject cooperated. It wasn't a dark mass. If the sun had peeked thru, it would have been a harsh mess, though.
In the photo below, I got lucky. The red barn is not a dark subject. It was also an overcast day. I was able to ignore the meter reading of the barn and expose for the snow. So, I overexposed by about 1.5 stops. You will notice that the snow has small areas of shadow which give the image depth. It's not the brightest white, but neither is overcast snow. Remember, we don't want a featureless mass of white.
A mass of white. That's not a photo I want to see. Which leads me to the choice of scenery. Break up the white with points of interest. In the shot above, the barn isn't the only subject. The snow is also the subject because of how it affects other objects in the scene. Without those objects the photo lacks a story.
In the shot below. of the grouping of trees, I broke up the white with the trees. The trees form a single subject even though they are many. Also, notice that the ground, sky and the snow on branches are all different shades of white. If I under or overexposed the snow these subtle details would have been destroyed and it would have become a white or muddy-gray mass.
You may have noticed that all of the reference shots are evenly illuminated and shot at moderate focal lengths. Be careful using ultra wide lenses with snow as they will capture large areas of varied exposure across the scene. Also be aware that long telephotos will usually have noticeable vignetting in monotone scenes.
Try to use your best lens for snowscapes. Lens vignetting will wreck havoc with your image as it will be very noticeable and cause processing errors, too.
Proper aperture choice is vital in snowscapes. In most cases, you'll need a large depth of field in snowscapes. In addition, most lenses perform best around f11. Try to shoot at f11 unless you have no choice otherwise. It is better to increase ISO up to 400 than to shoot at wide apertures. If you are using a zoom lens this becomes even more important.
Avoid close-ups in snowscapes unless the subject is strong enough to support the entire image. For example, a deer or a strong pattern or documentary statement.
If you are shooting wildlife in snow, do not prioritize the snow. In wildlife photography, the subject is always paramount. It's not worth loosing the shot to get the snow perfectly exposed.
Finally, remember that snow is white and therefore it is sensitive to color shifts. It also reflects the color of objects near it. Accurate white balance is vital in snow photography. Yes, there's plenty of white but the camera doesn't know which shade of white to balance for. Make a manual reading or be prepared to adjust your white balance in post processing. There's nothing worse than purple snow.