Powering the Raspberry Pi
The UART connector can also power the Raspberry Pi. However, do not connect two power sources! The two scenarios shown below are alternatives.
Raspberry Pi B+ powered via UART/serial cable:
In this case, you should not connect USB power.
Raspberry Pi B+ connected via UART/serial cable, but powered through USB:
In this case, you should not power the Raspberry Pi via UART (i.e. do not connect the red cable on the UART connector).
Similar image, for model B from http://elinux.org/RPi_Serial_Connection
In both scenarios above, the ethernet cable can be connected as well. The picture below shows Raspberry Pi B+ connected via UART/serial cable and ethernet cable, powered through USB:
If you remove the USB power supply, you could also power the Raspberry Pi using the red UART cable.