The RaspberryPi can be powered with a number of sources such as smartphone chargers or battery packs. The minimum requirement is that the source delivers 5V at 1amp , although there have been reports of adventurous geeks getting away with less (not recommended). With the processor pulling 750mA on ModelB rev2 there is not much left for GPIO use etc.
With the launch of the Model B+ the recommendation is that you use a charger / power source capable of delivering 5V at 2amps. The main reason for this is the addition of 2 new USB ports (4 in total) although the processor is now more efficient and draws less power (600mA). Again if you are NOT using all 4 USB ports the adventorous Raspbian might get away with using a lower power supply.