USB power , external power supply

What happens when my arduino board (decimalia) is connected via USB to my computer, and there is an external power supply also connected to the 5v pin. Does it draw the power from the USB or the 5v pin?

Does it draw the power from the USB

Yes, the 5V rail is taken from the USB connection unless you have an external power supply plugged in the jack socket, in which case it switches to that.