Icicles are formed when ice or snow is melted by either sunlight or some other heat source (such as poorly insulated building) and the resulting melted water runs off into an area where the temperature is below the freezing point of water (32 degrees) causing the water to refreeze. Over time continued water run off will cause the ice to grow.
These pictures I took driving around town. The top four came from one our oldest buildings in the village. It is a public building and does not have the best insulation. The rest are pictures of various houses except the last two. The last two are of the apartments I live in. When the buildings were remodeled a couple of years ago, these fantastic, expensive, icicles proof gutters were put on. You can see how fantastic they are. This may sound silly, but I like driving around looking at icicles.