Quartiles are values that divide a data set into four equal parts, helping to understand the distribution of the data. The first quartile (Q1) marks the 25th percentile, meaning that 25% of the data points fall below this value. The second quartile (Q2), also known as the median, divides the data into two halves, while the third quartile (Q3) represents the 75th percentile, indicating that 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. Quartiles are useful in statistics for summarizing data and identifying outliers, providing insights into the spread and central tendency of the data set.