Arctic cyclones are vast areas of low pressure. They should not be confused with what are commonly referred to as polar lows that behave similarly to hurricanes.
An arctic cyclone is a low pressure weather system usually spanning 1000-2000 kilometers in which the air is circulating in a counterclockwise fashion (in the northern hemisphere). The reason for the rotation is the same as tropical cyclones, the coriolis effect.
Cyclone activity is most prevalent in the Eurasian Arctic with approximately 15 cyclones per winter. Arctic cyclones also occur in Greenland and the Canadian arctic.
Arctic cyclones can occur at any time during the year. However, summer cyclones tend to be weaker than winter cyclones.