Disarming Charm

The spell Expelliarmus is a defensive spell that attempts to disarm your opponent, causing their weapon (usually a wand) to fly out of their hand. It has been known to knock an opponent backwards in some cases. Expelliarmus is shown to have a two color variety. In the books, it is described as a "jet of red light." But in the movie Harry Potter and the Goblet of Fire, when Expelliarmus is cast by Cedric Diggory, it is shown as blast of a bluish-white. It was aimed at Viktor Krum, who was thrown back and fell unconscious. Sometimes it has no color, just that it makes the wand fly out of the user's hand. When Professor Snape cast the spell on Gilderoy Lockhart, it was shown as a scarlet ring surrounding the wand, with a streak of scarlet light heading towards Lockhart.

It was first learned by Harry Potter by seeing Professor Snape using it in the Dueling Club that Gilderoy Lockhart started.

In Harry Potter and the Deathly Hallows, it is learned that the Death Eaters consider this spell to be Harry's "trademark", since he has performed this spell several times against Death Eaters and Voldemort himself, causing a Priori Incantatem. It is this spell that Harry performs against Voldemort in Voldemort's final duel. It is performed simultaneously against Voldemort's Killing Curse, but Voldemort was using the Elder Wand, which was rightfully Harry's. This caused both Harry's Expelliarmus and Voldemort's Avada Kedavra to hit Voldemort.

Trivia
In the Doctor Who episode 'The Shakespeare Code', the spell is mentioned in connection to the Harry Potter books, with the Doctor stating "good old JK" after Shakespeare says Expelliarmus and the Carrionites are vanquished from Earth.