A specific type of bar graph for showing the distribution of a single variable. The x-axis has subsets of the data range (classes), and the y-axis has the frequency that data points fall into each class. See the Wikipedia article for more information (e.g., on the different formulas for determining class breaks).

If you are at the beginning stages of data analysis, plot a histogram (and/or boxplot) to assess the structure of the data (e.g., if there are outliers, if there is high/low variability, if the data fall equally on either side of the mean, etc.). Take particular note of whether or not data are "normally distributed" (i.e., follow a bell curve). Many statistical tests requires data to be normal to function properly (e.g., the t-test).