Hello,
I am a newbee! Sorry! I am doing a sketch for my beautifull Arduino but...
I have made a board with 2 sides and I have for each sides let the PADS of the components...
My question: when I sold the bottom side PAD with a pin inside the hole, does the soldery goes into the hole making a VIA and solds itself also to the top PAD.
Else I don't know how to use the Eagle board skemes... it doesn't work.
Help meeeee!
Thanks in advance, Pierre. 
when I sold the bottom side PAD with a pin inside the hole, does the soldery goes into the hole making a VIA and solds itself also to the top PAD.
If you send the design to a PCB fabrication house, yes. The holes are known as "plated through holes", sometimes abbreviated as PTH. The hole for the component's pin can double as a via.
If you are making a board at home from double sided copper clad material, then no, the top and bottom will not be reliably connected unless you make sure to solder the pin on both sides.
-j