Melt-SnowAn ice dam is a ridge of ice that typically forms along the edge of a roof and prevents melting snow from draining off of the roof.  

Ice dams can also form around skylights, vents, and anywhere two inclined sides of the roof meet

Poor insulation and lack of adequate ventilation in the attic space causes the roof deck to heat up and melt the snow above.  When it does, the water ( melted snow) flows down the roof and gets backed-up behind the dam with nowhere to drain off.

This pool of water eventually refreeze into more ice (and continue to grow in size and the water can work its way under the shingles (or other roofing material).  At this point, the water can leak into the home or building, and quickly cause damage to walls, ceilings, insulation and other internal parts of the structure.