The four quartiles (Q1, Q2, Q3, and Q4) correspond to the 25th percentile, 50th percentile (also known as the median), the 75th percentile, and the 100th percentile respectively.  Each of the four groupings, or "bins", defined by these quartile values includes about 25% of the values represented in the dataset.  The bottom 25% of values lie below Q1, the next 25% lie between Q1 and Q2, the next 25% lie between Q2 and Q3, and the highest 25% of values between Q3 and Q4.

Grouping the data into these four bins helps to provide a measure of variability (or dispersion) of the data.