Duelling

A duel is a one-on-one confrontation betweeen two wizards or witches. The combatants face each other, bow, and then attempt to disarm, injure, or kill each other in order to force submission, and thus a winner is decided.

Rules
There are rules and customs that define how a proper duel is performed and that are normally known to wizards and witches born to magical families, even at a young age. Ron Weasley, for instance, told Harry Potter some of the rules of dueling as a first-year student at Hogwarts after Harry was challenged to a duel by Draco Malfoy in 1991.

The Challenge
"You've been taught how to duel, I presume? Yes? First we bow to each other... Come now, Harry, the niceties must be observed. Dumbledore would not want you to forget your manners, would he? I said, bow."

- Lord Voldemort to Harry Potter in the graveyard of Little Hangleton

In a formal duel, one wizard or witch challenges another, although it is implied that duels are not commonplace in polite society. If the other person accepts, they then arrange a meeting time and place for the duel. Opponents bow to one another before beginning to fight. Draco Malfoy challenged Harry this way in their first year, though it was really an attempt to get Harry into trouble for being out of his dormitories after curfew.

Seconds
Duelists will often have a "second" in a formal duel — a person, usually a trusted ally, who will act as a replacement for one duelist if necessary. Ron Weasley volunteered to be Harry's second in the aforementioned planned duel with Draco.

Limits
There is a standard rule in duels that only magical means may be used to fight an opponent. Millicent Bulstrode broke this rule when she dueled Hermione Granger in 1992 in the Dueling Club; the much larger girl ended up wrestling Hermione to the ground and holding her in a headlock.

The parties of a duel might also agree to other limits, unless planning to duel to the death, which is only common practice in a war. Minerva McGonagall, for example, informed fellow professor Horace Slughorn just before the Battle of Hogwarts that if any Slytherin attempted to sabotage their defence efforts or to fight them, they would "duel to kill". This may indicate that duelists generally agree not to use particularly dangerous and/or illegal spells, such as the Unforgivable Curses.

Practice Duels
Duels can also be used for training purposes, as was the case in Hogwarts in 1992, when Defence Against the Dark Arts professor Gilderoy Lockhart founded the short-lived Dueling Club. Its function was to teach students self-defence while "Slytherin's Monster" was on the loose. The club did not last long, as Professor Lockhart was incompetent.