With an average R-Value of 4, double glazed windows give you a high R-value while still allowing 76% light into the room. One of the two panes of glass will have a coating of Low E to reflect solar heat and to retain heat in the home during the winter months.
The third piece of glass and extra argon gas combined with LowE increases the R-value to an average of 6. The three panes of glass also reduce noise from the exterior. The down side to triple is that it blocks more light, only about 65% enters the home.