Any English speaker will obviously be able to tell you that there is a difference between the pronouns "he" and "she." But what about the pronoun "they?"
In French there are two words to express the concept of "they:" "ils" and "elles." The former is the masculine form of the word, which means that it would be used when one is referring to either a group of males or, since the masculine form is dominant, any group of people that includes a male. "Elles," on the other hand, refers to any group that is comprised of females.
So, the rule is:
a group of men = ils
a group of men and women = ils
a group of women = elles