I will focus on the popular, low-fat, high protein, and energy bars. There are also other types of protein, and a few other types of bars out there, which I have not reviewed.
The main difference between bars is their carbohydrate content. Low-fat energy bars are typically low in carbohydrates, and have an almost zero carb content. Energy bars are also often labeled with calories, which are typically much lower than the carbs in them, and may have a small amount of fat in them as well. Many people are concerned that low-fat energy bars have more sugar than their carbohydrate-rich counterparts. This is a misconception. The reason that carbohydrate-rich bars are higher in sugar is that they are mostly comprised of sugar. I have found that a good rule of thumb is that carbs in a protein bar are typically anywhere from 50-75% fat. This means that 50-75% of the carbs are fat, which usually has more calories than the rest of the bar. Protein is one of the more complex foods, and it takes a lot of energy to digest and process it. This means that if a person wants to eat a protein bar for breakfast, they are likely to gain weight throughout the day.