Quartiles[list] is equivalent to Quantile[list, {1/4, 1/2, 3/4}, {{1/2, 0}, {0, 1}}]. »
The second quartile is equivalent to Median[list]. »
For even Length[list], the first quartile is equivalent to the median of the smallest elements in list.
For odd Length[list], the first quartile is equivalent to the average of the median of the smallest elements and the median of the smallest elements in list.
The third quartile is defined as for the first, but with the largest rather than smallest elements.