Five-number summary
Five values are enough to capture a lot of information about the distribution of values in a data set.
These values split the data set into four groups with approximately equal numbers of values.
Box plot
A box plot displays the five-number summary graphically.
Details
The median, m, is the middle value if there is an odd number of values in the data set. If there is an even number of values, the median is the average of the middle two.
Different authors give slightly different definitions for the upper and lower quartiles. One definition of the lower quartile is the median of the lowest half of the data — i.e. of the values lower than m. (The upper quartile would then be defined as the median of the top half of the values.)
Provided you are consistent, different definitions of the quartiles should lead you to the same conclusions.