Solar Heat Gain Coefficient (SHGC):
The SHGC measures radiant heat that can pass through a window. Radiant heat is most easily noticed by the hot spots that one feels when he touches the floor where the sun has been shining. In northern states, a higher SHGC is better because it lets in more radiant heat. In Southern states, where keeping a house cool is more important, a lower SHGC is advantageous. For Virginia and other North-Central states, the best SHGC is between .25 and .35. 
According to EfficientWindows.org




Low-E Coating (SHGC):
Many windows today offer what is known as a Low-E or Low-Emissivity coating that significantly cuts down the amount of radiant heat that can pass through a window by adding a reflective layer to the inside of the glass. Low-e glass can significantly lower a window's SHGC and dramatically decrease cooling costs in the summer.