Suppose you want to order a pizza. The coupon you want to use will allow you to choose a large OR medium pizza, thick OR thin crust, and pepperoni OR mushroom topping. How may different types of pizza could you order?
One way to answer this question is to draw a tree diagram of all possible pizzas you could order. Then, if you count all of the "limbs" on the last row of the tree, you'll find the answer to the question.