Ice dams can cause major problems on some houses in the winter. An ice dam is the formation of a large mass of ice at the lower portion of the roof or in the gutters. If there is a large amount of snow on the roof and it begins to melt, water runs down the roof and gets trapped by the ice dam which can cause the water to back up under the shingles and into the attic or house. This can cause serious moisture damage to the house and is a problem that needs to be remedied.
To learn more about Ice Dams, Attic Venting, and Attic Moisture I recommend reading the following article from the CMHC: