Group functions in SQL are built-in functions that operate on groups of rows and return one value for the entire group. The document describes several common group functions: COUNT counts the number of rows that meet certain criteria; MAX and MIN return the highest and lowest values in a column; AVG calculates the average of numeric values; SUM adds up the values in a column; and DISTINCT eliminates duplicate rows. Examples are given showing how to use each function to analyze data from an employees table.