Resistor

Hello, I have put a 20mA LED with a 15ohm resistor in a breadboard, so it is possible damage my board? Or just the LED?

If the diameter of the resistor leads is too big you could damage the spring of the connection.
You are probably OK though.

LOL Larry,
You should use a larger resistor, like 100 or 1000 ohms.
But, you probably did not damage anything (unless, component is not working anymore).

15 ohm? That will let a lot of current flow from a 5V source.

Say the Vf of the LED is 2.5V, and the source is 5V. Then:
(5V - 2.5V)/15 ohm = 167mA
If the source can supply it, the LED will overheat and fail (such as from regulator, battery).
If the source can't supply it, the voltage will droop.
Or, the source could overheat and fail (transistor burnout in the Arduino processor).

@bl07, what is your question?

bl07:
I have put a 20mA LED with a 15ohm resistor in a breadboard, so it is possible damage my board? Or just the LED?

20mA LEDs have a forward voltage of about 1.8 to 2.2volt.
If you connect a 20mA LED and a 15ohm current limiting resistor in series to a 5volt Arduino pin, ~3volt must drop across the resistor and internal pin resistance.
The 15ohm resistor plus the internal pin resistance (25-50ohm) will draw 3/(15+25) = ~75mA from the output pin.
Almost twice the absolute maximum of 40mA.
If you're lucky, the LED will pop (open circuit) before you fry the Arduino pin.
Better use a 220ohm resistor.
That will limit current to 3/(220+25) = ~12mA.
Plenty for a 20mA max LED.
Leo..