You can use Arduino to make a commercial product following some simple rules.
if you have made your circuit as a derivative of the Arduino board you must release the design files with a CC-BY-SA license like the original cad files
If you build your circuit as a shield that plugs on top of an Arduino board all the circuit is yours and you don't have to release anything
The programs written on Arduino are yours. if you have modified the core files or one of the libraries you must make your modifications available to everybody
You can call your product in any way you like as long as you don't call it Arduino
Credits to Arduino not necessary
If in the documentation for your product you want to write "Powered By Arduino" that would be appreciated
There is no revenue sharing for any derivative work (unless it uses the Arduino name see Arduino - Home )
If you patent something you have to do that in every place in the world you want to protect your idea. the more countries the more payments
Patents reveal your idea to the whole world.. some people don't patent so they don't have to provide any detail about their invention