A portion is the amount of food that you choose to eat for a meal or snack. It can be big or small—you decide.
A serving is a measured amount of food or drink, such as one slice of bread or one cup (eight ounces) of milk.
Many foods that come as a single portion actually contain multiple servings. The Nutrition Facts label on packaged foods—on the backs of cans, sides of boxes, etc.— tells you the number of servings in the container.
For example, look at the label of a 20-ounce soda (usually consumed as one portion). It has 2.5 servings in it. A 3-ounce bag of chips, which some would consider a single portion, contains 3 servings.