25, 50 and 75% percentiles
The 50th percentile is the median and the 25th and 75th percentiles are the lower and upper quartiles. A box plot therefore shows these percentiles.
Displaying other percentiles
For some data sets, other percentiles are more important than the 25th and 75th ones. A similar 'box' can be used to graphically display any other percentiles. It is best to alter the way the box is drawn to avoid confusion with the standard box plot.