You use them to refer to a group of people, animals, or things.
The Beatles – I never get tired of listening to them.
甲壳虫乐队–我百听不厌。
Kids these days have no one to tell them what's right and wrong.
没人告诉现在的孩子们什么是对什么是错。
You use them instead of 'him or her' to refer to a person without saying whether that person is a man or a woman. Some people think this use is incorrect.
It takes great courage to face your child and tell them the truth.
需要很大的勇气来面对你的孩子并告诉他们真相。