In one way, there is no difference. Snowflakes
are ice crystals that are formed in the beautiful
and unique shapes that we are familiar with.
However, ice crystals in the shape of hexagonal
plates or columns, ice needles, dendrites, etc.
also form in (and compose) very cold cirrus clouds
and are responsible for producing sun dogs, some
halos around the Sun, and other atmospheric optical
Ice crystals also form on grass, trees, and just
about anything else as hoar frost on very cold
mornings, as hail in thunderstorms, in winter
they combine into small balls called graupel and
fall to the ground, and they produce icing on
airplanes in the wintertime.