Wind chill is a frequently misunderstood term. It is not actually a temperature scale but a measurement of heat loss. Here's how it works:
The basic law of thermodynamics says that any object warmer than its surroundings will lose heat. Normally we have an invisible layer of "still" air on the surface of our skin that acts like a blanket of insulation and slows our loss of body heat. Blowing wind reduces this insulating layer of warm air next to our skin and increases our rate of heat loss. The faster the wind blows, the more quickly we lose heat. It's one of the major reasons we feel cool when we stand in front of a fan in the summer (the other reason is evaporation, but that's another story).
The wind chill factor becomes critical when the air temperature drops below the freezing point. If wind is taking away heat faster than our bodies can replace it, we'll end up with a nasty case of frostbite. (Frostbite occurs when tissue freezes and dies). The danger of frostbite increases sharply as the air temperature falls and the wind speed climbs.
The combined heat loss from wind and low temperatures is called the wind chill factor. For example: An air temperature of 20º F when combined with winds of 15 MPH results in a heat loss rate equivalent to sitting in a freezer at 18 below zero. Brrrrr.
To use the chart you'll need to know two things. First: the actual temperature and secondly, a good guess at the wind speed. Find your wind speed in the left-hand column and then read across the row until you find the column that comes closest to matching the actual air temperature (listed in the first row of numbers across the top of the chart). The number you find in the box at the intersection of the wind speed row and the temperature column is the wind chill factor.