Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V).
5V.This pin outputs a regulated 5V from the regulator on the board.
You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin. VIN. The input voltage to the Arduino board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source).
