As mentioned before, sugars are simple carbohydrates.
Over the years, sugar has been demonized and blamed for the obesity epidemic.
Although sugar can be unhealthy, it's NOT what's causing people to gain weight. An overconsumption of calories (energy) is what's causing people to gain weight.
Now, this does not mean that we should continue to eat the way we have as Americans. The overconsumption of sugar (although not the primary cause of obesity) has definitely contributed to the epidemic in this country.
Sugar can give a false sense of hunger to our bodies, making us think we need to eat more which leads to an overconsumption of calories.
Not only can sugar make us feel hungrier, but sugary foods are not satiating, which makes it easier to eat more food without feeling full. For example. It would be much easier for us to eat 600 calories of Oreos than it would to eat 600 calories of whole food like meats, grains, etc.
Long story short, sugar alone will not cause you to gain weight, but it can contribute to unhealthy eating habits.