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.  This spell has a red color, and is frequently portrayed in the books as colliding with the green jets of Avada Kedavra. This spell has the ability to collide with other spells, therefore making both spells useless.
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. 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.
It's also called out as Harry's signature move in Harry Potter and the Deathly Hallows. Harry uses this spell many times with Lord Voldemory. It was this signiture move that enabled the Death Eaters to know who the real Harry Potter was in the Battle Over Little Whinging when seven different people took Polyjuice Potion to be disguised as Harry.