It's a program that controls motherboard voltages according to demand. It saves energy, but I'm using it mainly because it keeps the noise really low, especially at idle even with the stock cooler I have. I haven't noticed any performance issues yet, but I've only used it for couple of weeks.
ASUS EPU technology is a combo from a piece of hardware (EPU chip) and software EPU-4 Engine. I own an Asus mobo with EPU-6 engine and i used that utility when i don't overclock that machine.Does some energy savings. Here more infos: