Sorry for the unclear info.
You can use a (5V) charger to power via USB; you can in that case also power external components from the 5V pin. For an Uno, there is a fuse on the board to protect the USB port of the computer so there are limitations (500 mA total).
What you should not do is apply 5V to the 5V pin and at the same time apply 5V via the USB port.