An Ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow from draining off the roof. The water backs up behind the dam and can leak into a home and cause damage to walls, ceilings, insulation and other areas. An Ice Dam is usually caused when the attic temperature is warmer than the outside air. This generally is caused by not enough insulation or heat leaking from the house into the attic like around a light fixture or bath room fan.
Once you get an ice dam and have some leaking there is usually nothing you can do until it melts. If you get up on the roof and start chipping away, you run the risk of doing more damage than you would have had if you leave it alone until it melts. Not to mention damage to you if you fall off. If you do notice a spot on your ceiling that is starting to expand, it might be advisable to poke a hole in the dry wall and let drip down into a bucket rather than spread and the whole ceiling come down.
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