What happens if i put a tension on an output ?
I want to drive a 8x8 LED module. To light a specific LED (x,y), I will have to put row x on (OUTPUT, HIGH),
and the column y to GND. How can I do ? Can I set it to (OUTPUT, LOW) ? Is it possible to use a pull-down on an output ?
Thank in advance 
Yes, you can use an output LOW as "ground" and that is the correct way to set column y to GND.
To avoid damage to the output while it is LOW, you should not allow more than 20 mA to flow into it (use a current limiting resistor if this is a possibility, such is when you are lighting an LED).
Both need to be set as outputs, since you need to drive the row high and the column low.
It's not conceptually any different than lighting a led by putting one side on Vcc and connecting the other side to a pin and driving it low, or by putting the other side of the LED to ground and driving the positive side high. You're just doing both at once.
As always, it's important to make sure that you're observing the current limits of the pin - 40mA per pin max, 20mA recommended maximum. If you, for example, connected two pins directly (without an LED and resistor between them), set them as outputs and drove one high and the other low, or if you connected an output to Vcc or Gnd and then tried to drive it the opposite way, the current would be considerably higher, and it could damage the pins.
Perfect answers, it helps a lot. Thanks !!
And when you've done fiddling and if you want to use it seriously, get a module with a MAX7219.
