Quartiles are statistical values that divide a data set into four equal parts. They help to understand the distribution of data by indicating where the values fall. The first quartile (Q1) marks the 25th percentile, meaning 25% of the data points are below this value. The second quartile (Q2) is the median, dividing the data into two halves, while the third quartile (Q3) represents the 75th percentile, where 75% of the data points are below this value. To calculate quartiles, the data set must be arranged in ascending order. The quartiles can then be found using specific formulas or methods, such as the Tukey's method or inclusive method. Quartiles are useful in identifying outliers and understanding the spread of data, making them valuable in fields like statistics and data analysis.