Box plots have traditionally represented the five key measures of a variable in diagrammatic form. These measures are minimum value, maximum value, first quartile, second quartile and third quartile. Some like to see the mean added to this visualization (the diamond in the diagram below), and there are a number of variations on the theme. Below is a diagram of a box plot (often called a Box-Whisker plot).
The lower whisker end point is the minimum. The lower edge of the box is the first quartile and the upper edge is the third quartile. The middle line in the box is the second quartile, which is precisely the same as the median. The upper whisker end point is the maximum value of the variable. Overall it gives a good feel for the distribution of values a variable takes.
Users of Excel will know that the box plot is not supported. There are various ways of constructing a box plot in Excel and one such way can be seen by clicking here.
Excel users can expand the range of statistical tools by acquiring an add-in. One such free tool that includes the box plot is VTools (although a licence at US$5 adds further functionality).