Curses

Nature
The curse is a branch of evil magic known as "Dark Magic".

Curses are the most wicked, strongest, longest-lasting, and least reversible of the Dark spell types. The Unforgivable Curses are the strongest known Dark spells in existence, as their effects are very powerful and their use requires skill.

Usage
Like hexes, Hogwarts students learn only a few of them in school. However, students studying Defence Against the Dark Arts learn how to deal with them using defensive magic. Most curses can be blocked or lifted by a counter-curse, with the most notable exceptions being the Killing curse, the Breakup Curse and the Cruciatus Curse, three of the four Unforgivable Curses.