Warning: Spoilers Follow
Mundungus Flectcher was mentioned in passing in Harry Potter and the Chamber of Secrets (Arthur Weasley reports he tried to hex him when his back was turned) and twice in Harry Potter and the Goblet of Fire (Arthur, again, reports he demanded compensation for an alleged twelve-bedroomed tent with en-suite jacuzzi that was ruined during the Quidditch World Cup final, while in actuality he slept inside a cloak propped on sticks; Dumbledore also mentions him as a part of the "old crowd", obviously referring to the first assembly of the Order of The Phoenix, which needs to be assembled again). He remained unseen, however, until the first chapter of Harry Potter and the Order of the Phoenix. He was assigned guard duty to make sure Harry Potter is not attacked, but abandoned his position to conduct shady cauldron-trading business, leaving a critical opening through which the dementors sent by Dolores Umbridge managed to attack Harry and force him to use a Patronus, which was exactly Umbridge's plan. This earned him the deserved wrath of Arabella Figg.
Though Mundungus is a criminal, he is very loyal to Dumbledore, who apparently once got him out of serious trouble- apparently Dumbledore trusts him enough to include him in the Order of The Phoenix. He is described as a "squat, unshaven man" with "short, bandy legs", "long, straggly ginger hair" and "bloodshot, baggy eyes that gave him the doleful look of a basset hound". He also smokes a pipe which emits clouds of green smoke that smell distinctly like burnt socks.
Members of the order have mixed feelings about Mundungus- most notably Molly Weasley, who strongly disapproves of the constant dubious activity he conducts parallel to his membership in the order, and of the general idea of a part-criminal being included in the group; yet nobody denies his usefulness. Aside from selling Fred and George otherwise unattainable supplies necessary for the development of Weasleys' Wizard Wheezes, his connections enable him to hear things that others do not- rumors and information rolling around the shadier sects of the wizarding population which could potentially prove instrumental in the fight against Voldemort.