Citric acid is named for citrus fruit, in which it's quite prevalent. Lemon juice contains citric acid, but it also contains many other chemical compounds besides. Pure citric acid is common in nature, but is also a common food additive for its flavor and preservative properties. It has no important nutrient value.
Citric acid is very common in nature, and in fact, your own body cells make it. Your cells generate citric acid in the process of burning nutrient compounds -- fats, carbohydrates and proteins -- for energy. You break down the citric acid nearly as soon as you make it in the next steps of the energy-generating reaction process. While citric acid in food has a pleasant flavor, it has no significant nutrient value, and you can't use it the same way you use citric acid your cells make.
Lemons and lemon juice are one of the most concentrated sources of natural citric acid. According to Professor Glen Lawrence in his paper "Depleted Uranium and Health," a whole lemon contains about 3 grams of citric acid. This varies somewhat with the size of lemon and the cultivar, however. Citric acid in lemon juice contributes to the sour lemon flavor. Aside from citric acid, lemon juice contains many other things, including water, vitamins and minerals, and other sour flavoring molecules.
One of the most common misconceptions regarding citric acid is that it's the same thing as vitamin C. In fact, however, vitamin C is a completely different molecule that goes by the chemical name ascorbic acid. From a nutritional perspective, vitamin C is perhaps the most important component of lemon juice. You can't get vitamin C from pure citric acid. Furthermore, vitamin C helps you absorb iron, explains Dr. James Cook and colleagues in a 2001 article in the "American Journal of Clinical Nutrition." As such, lemon juice can help with iron absorption, while pure citric acid can't.
Because lemon juice contains compounds aside from citric acid, including sugar, it contains calories. Pure citric acid is very low in calories. This is because while citric acid is technically a calorie-containing food, you actually excrete most of the citric acid you consume, explains Dr. A. Pajor in a 1999 article published in "Seminars in Nephrology." Your cells take up an inconsequential amount of the citric acid you consume.