histogram is an approximate representation of the distribution of numerical data.  To construct a histogram, the first step is to bin or bucket the range of values. In other words, divide the entire range of values into a series of intervals and then count how many values fall into each interval. The bins are usually specified as consecutive, non-overlapping intervals  of a variable. The bins (intervals) must be adjacent, and are often (but not required to be) of equal size.

If the bins are of equal size, a rectangle is erected over the bin with height proportional to the frequency —the number of cases in each bin. However, bins need not be of equal width; in that case, the erected rectangle is defined to have its area proportional to the frequency of cases in the bin.The vertical axis is then not the frequency but frequency density—the number of cases per unit of the variable on the horizontal axis. Examples of variable bin width are displayed on Census bureau data below.

As the adjacent bins leave no gaps, the rectangles of a histogram touch each other to indicate that the original variable is continuous.

The histogram is one of the seven basic tools of quality.

Histograms are sometimes confused with bar charts. A histogram is used for continuous data, where the bins represent ranges of data, while a bar chart  is a plot of categorical variables. Some authors recommend that bar charts have gaps between the rectangles to clarify the distinction.

Reference : Wikipedia