Fred and George Weasley are the school jesters - not just class clowns but school clowns to boot. Rarely serious, they enjoy playing Quiddich (as beaters on the Gryffindor side), making practical jokes, irritating Percy and, er, irritating Percy. And irritating Percy. And then some more.
Ahem. Anyway, they are identical twins and have identical tastes. They are practical jokers and are really very different from Percy (and ron). Mrs Weasley constantly screams at them because of their practical jokes (in the first book they attempt to send Harry a toilet seat when he is in hospital). And they are world class gnome throwers (at least they'd like to think so).
In the third book they reveal to Harry a secret map of Hogwarts they filched from Filch's cabinet of highly dangerous banned objects. Which, of course, turns out to be the famous Marauder's Map. Having already memorised it, they are privy to all the secrets of the castle, from the secret entrance into Hogsmeade to the entrances into the kitchens of the school. They are great favourites with the House Elves (who Hermione later attempts to liberate) and great unfavourites of Filch and Mrs Norris.
They aren't particularly good in school, but that really isn't such a bad thing. They are good at Quiddich and can predict the outcome of Quiddich matches with some certainty (they win their bet with Ludo Bagman in Book 4). At the end of Book 4 they plan to open a practical jokes store, and create order forms for Weasley's Wizard Wheezes. All in all, they are good fun.