Knowing your skin tone and undertone is very important for your makeup. They are the reason why some people end up with a foundation that is too pink on them for example. In this case, they got a foundation with pink/cool undertone while they have yellow undertone.
To know what is your undertone, you can just take a look at the veins in your wrists :
Another test is just to pull your hair off and drape a white piece of cloth around your neck and look at your face in the mirror. The contrast of colors usually shows the undertone of your skin.
Last one is the jewelry test. If you look good in gold, you are warm and if you are better in silver you have cool undertones. If you look good in both, than you are lucky to have neutral or olive undertones.
The chart below chart (bareMinerals website) is a good matrix of different skin tone (light, medium, tan, medium dark and dark) with their undertone (neutral, cool, golden, warm).