Fred and George are the school clowns, more interested in making new jokes than their studies. They are the beaters on the Gryffindor Quidditch team. Much to their mother's disappointment, they are the only of her sons who weren't made prefects.
In Harry Potter and the Goblet of Fire, Fred and George begin selling their own jokes by mail order, under the name "Weasley's Wizardly Wheezes". They help Harry to get some revenge against his first cousin Dudley Dursley by "accidentally" dropping several Ton Tongue Toffees—Dudley, a glutton on a forced diet, soon has a foot-long tongue protruding from his mouth; at the Quidditch World Cup they impress Ludo Bagman, Head of the Department of Magical Games and Sports at the Ministry of Magic with their fake wands.
Despite their mother's wish that they should follow their father in working at the Ministry of Magic, their ambition in life is to run their own joke shop. Harry makes this possible by giving them his winnings from the Triwizard Tournament. Apparently they now have one in Diagon Alley that they started working at after they escaped from Umbridge.
The pair provide Harry with useful assitance throughout the series; in Harry Potter and the Prisoner of Azkaban they give him the Marauder's Map, and in Harry Potter and the Order of the Phoenix they provide distractions so that Harry can use the Floo system unmonitored in Dolores Umbridges' office. They were members of Dumbledore's Army, a group started by Harry, Hermione and Ron to provide the practical instruction in Defense Against the Dark Arts, which Dolores Umbridge had removed from that course's curriculum.