A portion is the amount of food that you serve yourself for a meal or snack, like when you pour a bowl of cereal or get a scoop of potato salad. A serving is a measured amount of food or drink, such as one slice of bread or one cup of milk.
Many foods that appear to be a single portion actually contain multiple servings. I made this mistake recently with a can of Campbell's Tomato Soup. It's a very small can so I "assumed" it was a single serving but low & behold, it was actually 2-1/2 servings! It's simple to see how easily it would be to overeat and perhaps not even realize it. Fortunately, the USDA is re-evaluating serving sizes to be more realistic to what a consumer would buy (whole servings) and what a consumer would eat.
Always check the nutrition facts labels to determine the appropriate serving size!