Five-number summary
The distribution of values in most data sets can be effectively summarised by a few numerical values called summary statistics.
Box plot
The box plot of a batch of values displays these five values graphically.
Details
We have skipped over some details in our description of the median and quartiles. You should usually rely on a computer to evaluate them, so a precise definition is not strictly necessary. The idea of splitting the data into 4 equal-sized groups allows you to interpret the shape of box plots.
Definitions of median and quartiles
For those interested, here are some details about calculating a data set's median and quartiles.
Provided you are consistent with your definitions, the box plots that you will draw should lead you to the same conclusions about the differences between groups.