What is a Resistor?
How to read a resistor
The color bands on a resistor allow you to determine the resistance of the resistor. The first band determines the first digit of the resistance. The second resistor specifies the second digit. The third digit is the multiplier. So you take the first and second digit, say 47, and you take the multiplier, and multiply the number by the multiplier.
The forth band is for the tolerance. The tolerance is the range of valid resistances for that resistor. This is included to specify the maximum variations from manufacturing.
Using a Resistor
Unlike some electronics components like LEDs, resistors are not polarized. This means that current can flow through them in either direction.
Just like most electronics, resistors have a limit to how much power they can handle. Most resistors are limited to 0.25 Watts. If you want to know if you are safe to use a specific resistor in a circuit you can use P=V2/R, where P is the power in watts, V is the voltage drop, and R is the resistance of the resistor. Since calculating the voltage drop requires a more in-depth understanding of electronics, you can calculate that for 5V systems like the Arduino, any resistor with a resistance of 100Ω or more is safe. However we do encourage you to learn more about electronics, and learn how to check the power through each resistor.