Quartiles for a data set( arranged in ascending
order) is the three data points that divide the set of data
into four parts, each containing a quarter of the data.

Example:

The first point marks the lower quartile(Q1)
boundary at the 25th percentile.

The second marks the middle quartile( Q2
) at midpoint of the data set, the median or the 50th percentile.

The last marks the upper quartile( Q3
) at 75th percentile.