"Language, Weasley..."
     -- Draco Malfoy

Like people everywhere, Wizards pepper their speech with slang expressions and interjections. Some of these are the same as those used by their Muggle counterparts, but others are more specific to the Wizarding World. Some are even specific to the particular characters and their shared experiences (e.g. "pull a Crouch.")

There are a few words included which are considered swear words, including "damn" and "hell," but the worst of such language is not actually transcribed in the text. It is alluded to, presumably to keep the books appropriate for younger readres. Ron seems to be particularly in the habit of using bad language.

Ron caught up with them five minutes later, in a towering rage.
"D'you know what that --" (he called Snape something that made Hermione say "Ron!") "-- is making me do? I've got to scrub out the bedpans in the hospital wing. Without magic!" He was breathing deeply, his fists clenched. 

Ron told Malfoy to do something that Harry knew he would never have dared say in front of Mrs. Weasley.

“Oh Professor, look! I think I’ve got an unaspected planet! Oooh, which one’s that, Professor?”
“It is Uranus, my dear,” said Professor Trelawney, peering down at the chart.
“Can I have a look at Uranus too, Lavender?” said Ron.
Most unfortunately, Professor Trelawney heard him, and it was this, perhaps, that made her give them so much homework at the end of the class.

"pull a Crouch" - Ron (GF
Ron is suggesting with this phrase that Percy might betray his own family in the name of following rules, in the same way that Barty Crouch Sr. disowned his son with barely a fair trial.

"Gallopin' Gorgons" - Hagrid (SS4)
A Gorgon is a creature from Greek mythology (and possibly, in the Wizarding World, a real creature). Here's what the Encyclopedia Mythica has to say about them:

In Greek mythology a Gorgon is a monstrous feminine creature whose appearance would turn anyone who laid eyes upon it to stone…The Gorgons are monstrous creatures covered with impenetrable scales, with hair of living snakes, hands made of brass, sharp fangs and a beard. They live in the ultimate west, near the ocean, and guard the entrance to the underworld. (http://www.pantheon.org/articles/g/gorgons.html)

"working like house-elves" - Ron (much to Hermione's chagrin) (GF14)
Hermione takes offense at Ron's casual reference to what she sees as a gross injustice. Ron is comparing how hard he and Harry have been working to how hard the House Elves of Hogwarts work to maintain the castle.

"like some common goblin" (GF8)
used by Winkey to describe Dobby's unbecoming behavior:

"...You goes racketing around like this, Dobby, I says, and next thing I hear you's up in front of the Department for the Regulation and Control of Magical Creatures, like some common goblin."

"eat dung" - Ron to Malfoy (GF11)
Essentially no less rude than the more common expression which wouldn't be used in a children's book. 

"eat slugs" - also Ron to Malfoy, shortly before the Slug Curse backfired (CS
Possibly using this insult gave Ron the idea for the Slug Curse he used a little while later. "Eat slugs" isn't the actual Slug Curse, of course, it's just a slan expression.

"obsolete dingbat"
Rita Skeeter called Dumbledore this in an article for the Daily Prophet (GF

"lousy, biased scum-bag"
Ron, about Karkaroff's low score for Harry's performance on the first task (GF20)

"miserable old bat"
Ron again, this time talking about Professor Trelawney after she assigned a ton of homework (GF
 

see also
nicknames