Quantiles are values that divide a data set into equal-sized groups. For example, the median is a quantile that divides a data set into two equal-sized groups. Other quantiles include the first quartile (which divides the data set into four equal-sized groups, with 25% of the data below and 75% above), the third quartile (which divides the data set into four equal-sized groups, with 75% of the data below and 25% above), and the percentiles (which divide the data set into 100 equal-sized groups, with each percentile representing a certain percentage of the data below it). Quantiles are useful for understanding the distribution of a data set and for comparing different data sets.