There are many variables that describe snowpack and which you can explore at this website.
Couldn't be simpler. The distance from the top of the snowpack to the ground is the snow depth Hs. This is measured with a meter stick, a tape measure, an avalanche probe, or other device.
Snow Water Equivalent (SWE)
SWE is simply the amount of water you would get if you took a vertical column of snow and melted it. By definition SWE must be less than snow depth Hs.
Snow density is just the ratio of SWE to Hs. It ranges from 0 to 1, with 0 being air (think fluffy powder snow) and 1 being liquid water. It's essentially a measure of the weight of the snow.
Snow Covered Area (Sentinel imagery)
Satellite imagery (Sentinel, Landsat, other...) provides high-resolution information on the extent of snow cover. This example is Mt. Hood, Oregon, during late season conditions.
Snow Cover (MODIS reflectance)
Snow cover can also be 'inferred' based on reflectance. Snow has high visible-range reflectance, but low shortwave infrared reflectance. The 'normalized difference snow index' (NDSI) is an indicator of snow cover.