Detentions
"That's how it is at Hogwarts. Copyin' lines! What good's that ter anone? Yeh'll do sommat useful or yeh'll get out."
     -- Rubeus Hagrid

"It's just a pity they let the old punishments die out...hang you by your wrists from the ceiling for a few days, I've got the chains still in my office, keep 'em well oiled in case they're ever needed..."
     -- Argus Filch

detentions
points

In years past, punishments for rule-breaking at Hogwarts were severe indeed. Filch speaks longingly about being allowed to suspend guilty students by their wrists from the ceiling for a couple of days (SS15). He still begs Dumbledore for the chance to hang a student up by the ankles (CS8) and keeps a set of chains all polished up and hanging from the ceiling in his office. Molly Weasley recalls the night when she and Arthur were caught out of their common room taking a moonlight stroll; she escaped capture, but Arthur was caught and punished by the caretaker, Apollyon Pringle, and "still has the marks" (GF31).

Nowadays, the punishments do not typically involve physical torture, but nevertheless they can be extremely unpleasant. Filch, for example, was ready to lock Harry and Ron in the dungeons on their very first day when he caught them trying to open the door into the forbidden third floor corridor and wouldn't believe that they were lost (SS8). Teachers are given the freedom to choose the type of punishment they mete out, and Snape makes good use of this power to make life miserable for miscreants. Professor Moody overstepped this power, however, when he used Transfiguration to turn Malfoy into a ferret and bounce him around the Entrance Hall (GF13).

Punishments vary according to the nature of the crime. Some infractions result in points being deducted from the offender's house. For other more serious offenses, detentions are handed out. Teachers may assign detention tasks as they wish and some detentions have been actually dangerous. If a student refuses to accept the detention, they will be expelled. Large hourglasses show the number of House Points each house has accumulated (SS15).

detentions:
search the Forbidden Forest with Hagrid in the dead of night
who: Harry, Hermione, Neville, Draco
why: wandering around the castle in the middle of the night
issued by: McGonagall
points taken: 20 from Slytherin, 150 from Gryffindor
ref: SS15
polish the silver in the trophy room without magic
who: Ron
why: coming to Hogwarts in a flying Ford Anglia
issued by: McGonagall
points taken: none (school wasn't technically in session yet)
ref: CS7
help Lockhart answer fan mail
who: Harry
why: coming to Hogwarts in a flying Ford Anglia
issued by: McGonagall
points taken: none (school wasn't technically in session yet)
ref: CS7
disembowel a barrel full of horned toads
who: Neville
why: melting his sixth cauldron
issued by: Snape
ref: GF14
clean out the bedpans in the hospital wing without magic
who: Ron
why: criticizing the way Snape taught
issued by: Snape
ref: PA9
unnamed detention
who: Marcus Flint, Malfoy, Crabbe, and Goyle
why: pretending to be Dementors to make Gryffidor lose a match
issued by: McGonagall
points taken: 50 from Slytherin
ref: PA13
unnamed detention
who: Neville
why: losing a list of the passwords to the Gryffindor common room, a list which Sirius Black got a hold of and used
issued by: McGonagall
ref: PA14
pickle rats' brains in the dungeons
who: Harry and Ron
why: yelling at Snape
issued by: Snape
points taken: 50 points from Gryffindor
ref: GF18