GlacialTech is known for their light and cool materials. Both the Igloo Silent and PWM are just that, with superior noise reduction. The Silent has a 3-pin connector and the PWM has a 4-pin, meaning that the PWM takes advantage of the newer CPU fan connectors on the latest motherboards, allowing the motherboard itself to control the speed of the fan. In short, when things get hot, the Igloo PWM gets going. When temperatures are back to normal, the motherboard sends a signal through the fourth wire that tells the fan to slow down. But fear not if you only have a 3-pin on your mobo, either one will still work on older and newer motherboards.

http://www.ocmodshop.com/ocmodshop.aspx?a=1154