Blizzard refers to the conditions during a snow storm, when snow is falling heavily, and is generally being whipped around by strong winds. It is also used to describe an activity or situation with similarly chaotic action, such as being 'buried under a blizzard of email' (much as we describe a 'whirlwind of activity').
An extreme form of blizzard is a white out, where the snowfall is so severe that it is impossible to distinguish the ground from the air. People caught in a white out can quickly become disoriented, losing their sense of up and down as well as their sense of direction.
Blizzard is also the name of a PC video game company famous for making the following games: