A bell curve is a type of graph that is used to visualize the distribution of a set of chosen values across a specified group that tend to have a central, normal values, as peak with low and high extremes tapering off relatively symmetrically on either side. Bell curves are visual representations of normal distribution, also called Gaussian distribution.
A normal distribution curve, when graphed out, typically follows a bell-shaped curve, hence the name. While the precise shape can vary according to the distribution of the population, the peak is always in the middle and the curve is always symmetrical.
Bell curves are useful for quickly visualizing a data set's mean, mode and median because when the distribution is normal, the mean, median and mode are all the same.
The long tail refers to the part of the bell curve that stretches out in either direction. If the diagram above represents a population under study, the fat area under the bell curve is where most of the population falls.