Snow is made of flakes piled together. Each flake is a 6 sided circle; each one is supposedly unique in the details of its crystal arrangement. Some frosted flakes are shown in the figure. Snow crystals form from condensation of atmospheric water vapour. However, the vapour freezes into snow flakes instead of rain drops when the air temperature is sufficiently low, below 32°F (0°C). The crystalline flakes are very light weight since there is a lot of space between the water molecules making up the crytals, but still much denser than air, so it falls from the clouds to the ground. Typically, fresh snow on the ground is about 9 times less dense than water. The snow scatters sunlight very well, so it looks white.
Despite the fact that snowflakes typically fall when the temperature in both the clouds and the ground is 32°F (0°C) or lower, snow can fall at temperatures above freezing level and rain can equally fall below freezing level. High humidity levels often causes more wet snow and makes it more likely to fall as rain or sleet, even when below 32°F. Low humidity levels have the opposite effect, making it more likely to snow at temperatures several degrees above freezing. Another factor would be the dew point - a lower dew point causes for snow at higher temperatures, and a higher dew point causes for more wet snow at a potentially lower temperature.