Muggles' Guide to Harry Potter/Magic/Silencio

Overview
Silencio is used to silence an object or a person.

Extended Description
We see this charm being taught to the Trio in Harry Potter and the Order of the Phoenix. Hermione is easily able to silence her raven; Harry and Ron have trouble with the toad they are supposed to be silencing, and are unable to silence Hermione's raven after they swap.

Later, Hermione silences a Death Eater who is calling out that he has found her, Harry, and Neville during the Battle at the Department of Mysteries.

Analysis
There is some question as to whether this spell breaks the rule that defines Charms. According to the author, Charms are those spells which do not change the basic characteristic of an object. Transfiguration, therefore, is not a Charm as it involves changing objects into other objects, or making them vanish. And yet, the class where Silencio is being taught is Professor McGonagall's Transfiguration class. Does silencing a raven change its essential raven nature? If not, this spell should be taught by the Charms instructor, Professor Flitwick.

There is some thought that, in silencing the Death Eater at the Department of Mysteries, Hermione may be attempting to prevent his casting more spells. This does not, in fact work, as the silenced Death Eater is still able to cast a disabling spell on Hermione. Casting spells silently is a skill that starts to be taught in sixth year, and at the time, the Trio are in their fifth year of school.