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 resistor bands are read in a left-to-right fashion with the tolerance band (usually offset from the other bands and a gold/silver color) on the right. To read a resistor's color code, start with the band furthest to the left; this will be the first, most significant digit. The next band to the right is the second, least significant digit. The third band is the multiplier, and the fourth band is the manufacturing tolerance level.
For instance, assume there is a resistor with the bands Purple, Yellow, Orange, Gold. To read this resistor's value, start with the band furthest from the the tolerance band (in this case, purple) and check its value on a resistance color code chart (a value of 7). We do the same thing with the next band (yellow, with a value of 4) and append that value to the first band's value (74). Then we check the third value against the multiplier chart (in this case orange, so a value of 1000 or 1k; an easy way to remember this is 'how many 0's will be appended to the resistor value'. Since orange has a value of 3 on the resistor color code chart, three 0's will be added to the resistor value). The resistor now has a value of 74,000 or 74k. Finally, the tolerance band denotes what range of values the resistor should fall under due to manufacturing tolerances; since this band is gold, it has a tolerance of +- 5%. This tell us that the resistor should have a value ranging from 70,300 ohms (70.3k ohms) to 77,700 ohms (77.7k ohms)
The true value of a resistor can then be calculated using a multimeter or an ohmmeter. Note that while there are small-value resistors (lower than 100 ohms), extremely large value resistors (greater than 100 Megaohms) and low tolerance resistors (as low as +-1%), most resistors fall within 100 to 1,000,000 ohms (1 million ohms is 1 megaohm, or 1Mohm) and have a tolerance value of +-10% or +-5%.
Using a Resistor
Unlike some electronics components (such as 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.