Quartiles divide a data set into four equal parts. The first quartile (Q1) is the median of the lower half of the data. The second quartile (Q2) is the median of the entire data set. The third quartile (Q3) is the median of the upper half of the data. To find the quartiles, the data must be sorted in ascending order and the values that divide it into four parts are the quartiles. The interquartile range is calculated as Q3 - Q1 and measures the spread of the middle 50% of the data. Examples are provided to demonstrate calculating quartiles and the interquartile range.