Temperature, humidity shape snow crystals.
As snow crystals form they take on a six-sided, or hexagonal shape, but with what seems like an infinite number of variations of being six sided. The temperature at which a crystal forms, and to less extent the humidity of the air, determine the basic shape. The many things that happen to snow crystals as they fall, such as collisions, partial melting and colliding with water drops that freeze to them, create even more shapes. This is why irregular crystals with no easily identifiable form are the most common. Some times crystals are a combination of more than one form. For example, hollow columns that form in air colder than -8 Fahrenheit could grow thin plates on one or both ends as they fall through warmer air. While most people refer to shapes like those in the graphic above as snowflakes, flakes are really made of many snow crystals that have stuck together.

Snow crystals form hexagonal shapes because of the way the two hydrogen atoms that join with an oxygen atom to form a water molecule attach to the hydrogen atoms of other water molecules.