An Arduino Uno can be powered by
- a stable (regulated) 5V DC, which you can either supply via the USB power lines, or via the shield connectors, or
- an unregulated 6-20 V DC (7-12V recommended), which you can supply via the 2.1 mm centre-positive barrel plug connector.

I would not recommend using a 9V battery, because its voltage will drop quickly, but in a pinch this can be used.

ref: http://arduino.cc/en/Main/ArduinoBoardUno