A portion is what you actually serve yourself. A serving is what the math says you ought to serve yourself, based on nutrition guidelines for healthy women and men. At Cooking Light, we use those guidelines (2,000 calories per day for women and 2,700 for men under 50) to set serving sizes for snacks and meals. Similar math drives the serving sizes you see on food packages.
Rough as the math is (desk potatoes and marathoners have vastly different requirements), it's obvious that many Americans eat portions that are bigger than the recommended serving sizes. This isn't because they can't guess what a serving looks like: When we told 25 volunteers the amounts that approximate a healthy dinner of pasta, garlic bread, and salad, the majority were able to dish out the correct portion onto a plate (Plate X). However, 70% admitted they would serve themselves more (Plate Y).
Closing the portion--serving gap poses a huge challenge at the national level. At the personal level, the "cure" is simple: Know thy serving sizes; know where calories lie; favor whole, low-calorie foods (more salad); and measure portions until your personal gap narrows.