Normal (commonly referred to as bell) curves are symmetric, because if you draw a line vertically through the mean of the distribution there is as much of the distribution on the left as there is on the right. If a distribution has more of the observations on the left side of the mean it is called right-skewed (because it has a tail skewing off to the right), and visa versa if a distribution has more of the observations on the right side of the mean it is called left-skewed. Watch this video also -Normal Distribution