Here is a good explanation of the information found on nutrition labels required on all packaged food sold in the United States. I see very few people looking at the labels when shopping. Most seem to only look at sodium content, which is good, but serving size is also important. I once saw a product with several hundred milligrams of sodium, which I thought was low until I saw a serving size of two tablespoons.

Source: How to Understand and Use the Nutrition Facts Label | FDA