They're actually fairly pretty, and many ice dams look like white frosting just dripping off the house. But looks can be deceiving.
Ice dams are those thick ridges of ice hanging from your home's eaves, gutters or siding. When ice dams form, they can weigh down and potentially break gutters. Ice dams can also block melting snows from properly draining and can lead to roof leaks. The common thinking is that the closer the ice dams are to the house, the more troublesome they can be to your home.
According to ENERGY STAR®: warm air inside the house escapes into the attic if not properly insulated. This will warm the underside of the roof, causing snow and ice on the roof to melt. The melted water will drain along the roof, under the snow, until it reaches the cold overhang. The overhang tends to be at the same temperature as the outdoors and the melted water will refreeze and form an ice dam and icicles. The ice dam can cause damage to the roof, which will result in water leaks to the inside. Frequently the result will be a water spot on the ceiling under the roof damage.
Taking a shovel and knocking off overhanging icicles isn't the solution. Neither is pouring hot water on the ice. Click HERE for a prescription checklist from ENERGY STAR that gives tips on handling ice dams and how to prevent them from reoccurring on your roof.