First mentioned in Harry Potter and the Chamber of Secrets, when Snape uses the charm to disarm the then-Defense Against the Dark Arts professor, Lockhart. This charm disables the castor's opponent, which has been portrayed as something as simple as knocking a wand out of someone's hand in Harry Potter and the Chamber of Secrets to something as unique as the phoenix cage in at the end of Harry Potter and the Goblet of Fire. It is also known as the Disarming Charm. 
The Expelliarmus spell is the first spell that Harry teaches to Dumbledore's Army. Zacharias Smith questions Harry saying that the spell will not be useful against You-Know-Who. Harry then tells the group that he did use the Expelliarmus spell against Voledmort and it saved his life.