As we all know, electricity moves as waves. There are different types of waves (http://en.wikipedia.org/wiki/Waveform). Different power supplies send electricity in different forms. For instance, some UPS devices supply electrical waves in the form of square waves and not all devices can function on such waves. What waveforms does the Raspberry Pi accept?
As Marco Poli said, the raspberry Pi takes 5V DC input from it's power source (normally a USB charger connected to the Micro-USB power-in port). DC should be flat (no waves) the charger, or other power supply that you use might care about sine wave vs square wave on the AC source, but by the time it gets to the RPi it should be flat old 5V DC. The RPi team decided on 5V over a micro USB because that allows you to very easily find a power supply that works. (a used phone charger)
What you do need to watch for is amperage. >300mA for a model A, and >700mA for model B.