What Causes Ice Dams?
As heat escapes, it melts snow and ice on the roof. The melted water drains along the roof, under the snow, then refreezes when it reaches the roof's cold overhang. The resulting icicles are called ice dams because they trap melting water and keep it from draining off the roof. The water has to go somewhere though - which is why you get leaks through your walls and ceilings. Ice dams can damage your roof, but they aren't a roofing problem.
See this Ice Dam presentation by Horch Roofing and Evergreen Home Performance to learn more: