Hi everyone
I'm new to electronics and Arduino, but I'm already having a lot of fun with my little starter kit 
As I said, I've never done any electronics before. I only studied electricity somewhat at school. And... I don't really understand the purpose of resistors. For example, why should I put a resistor between a LED and the ground ?
I'm willing to read lots of documentation on the subject, but I didn't find good ones. Any explanations or links ?
Thx for help
An LED is a nearly resistance-free device. Therefore, when plugged in to the Arduino, it acts like a short circuit. As you might imagine, that is not a good thing. The Arduino is not able to supply an infinite amount of current.
The purpose of the resistor is to restrict the amount of current that flows through the resistor into the LED, to a level that the Arduino CAN supply. Google Ohms law for more than you ever wanted to know.
Resistors are a part one of the most fundamental concepts that should be understood related basic electronics. It is known as "Ohms Law".
Hey, thank you for answering so fast I'm going to google that right now
This explains why you must have a resistor with an LED.
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html