The power pins are as follows:
+VIN. The input voltage to the Arduino board (i.e. the same as the V+ screw terminal). You can supply voltage through this pin, or, if supplying voltage via the screw terminals, access it through this pin. Warning: The protection for reverse polarity connection is ONLY on the screw terminal, do not attach negative voltages to this pin. It will damage the board.
5V. This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the screw terminal (2.5V - 12V) or the VIN pin of the board (2.5V-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
GND. Ground pins.