Muggles' Guide to Harry Potter/Magic/Parselmouth

Overview
A parselmouth is a wizard or witch with the ability to understand and talk to snakes (using Parseltongue).

Extended Description
Perhaps the best known Parselmouth was Salazar Slytherin. His heir, Tom Riddle, possesses this ability as well, and presumably passed it on to Harry Potter via his attempt to kill him with the Killing Curse. Parselmouths are often assumed to be evil, partially because of their association with Voldemort.

In Harry Potter and the Half-Blood Prince, three more Parselmouths are introduced: Marvolo, Morfin and Merope Gaunt. We learn at the same time that Professor Dumbledore understands Parseltongue, and presumably can speak it, though he never does so in Harry's hearing.

Greater Picture
It is initially uncertain whether Voldemort is aware that Harry is a parselmouth, though likely rumours have reached him. The incarnation of Voldemort that is aware of Harry's ability was destroyed at the end of Harry Potter and the Chamber of Secrets, and Professor Dumbledore suggested in Harry Potter and the Half-Blood Prince that Voldemort was likely unaware of its destruction at the time. He may have since learned of its fate from Lucius Malfoy, but will not have learned any fine detail from that source, since we discover in Harry Potter and the Deathly Hallows that Voldemort is unable to sense the destruction of the diary, or anything else about it. It is also unknown whether Snape would have mentioned Harry's being a Parselmouth to Voldemort, given his complex loyalties. So having an opponent who can unexpectedly understand and countermand his orders to a part of his army might prove a problem for Voldemort.

As it turns out, however, the final battle does not involve any significant number of snakes, apart from Nagini; so while the fact that Harry is a Parselmouth is important to the story in the seventh book, it is not a factor in the final battle. By Christmas of Harry's seventh year, however, it is certain that Voldemort does know that Harry is a parselmouth; the trap that Voldemort sets for Harry in Godric's Hollow will not work on anyone who does not speak Parseltongue.