Muggles' Guide to Harry Potter/Magic/Wingardium Leviosa

Overview
Wingardium Leviosa is a common spell used to levitate objects.

Extended Description
This is the first charm taught to Harry's first year class; it is a relatively simple charm, but only Hermione is able to perform it correctly at first.

It is explicitly mentioned at three other places in the series.

Firstly, during the same evening it is introduced, Ron uses it to levitate a club above the head of an attacking mountain troll; when he releases the spell, the falling club knocks out the troll.

Secondly, in Harry Potter and the Deathly Hallows, Harry uses it on a sidecar which has fallen off a flying motorcycle. In fact, Harry is actually in the sidecar when he uses Wingardium Leviosa on it; while it seems that the ability to fly unaided is exceptional among wizards, the ability to charm objects on which you are mounted is relatively simple.

Thirdly, in chapter 32 of the same book, Ron uses the spell on a fallen-off twig of the Whomping Willow to touch the knot on the tree's trunk that will stop its branches from violently attacking anyone near the tree. The Trio then enters the tunnel that leads from the Willow to the Shrieking Shack.

Wingardium Leviosa is also mentioned implicitly in the chapter about Harry's O.W.L. exams in Harry Potter and the Order of the Phoenix, where it is the answer to the first question Harry reads in his theoretical Charms exam. Harry instantly remembers the defeating of the mountain troll in his first Hogwarts year mentioned above, and therefore the answer comes easily to him, which greatly reduces his initial nervousness.