|Muggles' Guide to Harry Potter - Magic
|The ability to talk to snakes
|Harry Potter and the Philosopher's Stone (first appearance); Harry Potter and the Chamber of Secrets (named)
Parseltongue is the language of snakes. A wizard capable of speaking Parseltongue is called a Parselmouth.
Extended Description edit
The ability to speak Parseltongue is usually inborn; a wizard is born being able to speak Parseltongue, and those who are not born with the ability cannot normally learn the language (although, according to the author, Dumbledore may have learned to understand it, and in Harry Potter and the Deathly Hallows, Ron learnt one word of it, though it is questionable whether he knows what it means). To date, the ability to speak Parseltongue has been almost entirely associated with those who can show direct descent from Salazar Slytherin.
Harry, it turns out, can speak Parseltongue, though he was apparently not born with that ability. In Harry Potter and the Chamber of Secrets, Professor Dumbledore suggests that when Lord Voldemort, who is Slytherin's last descendant, tried to kill Harry, some of Voldemort's powers were transferred to him; one of those was Parseltongue.
Harry's ability to speak Parseltongue is first described in Harry Potter and the Chamber of Secrets; it is only there that we discover how rare this ability is and how it is linked to Voldemort. However, this is not the first use of this ability; in Harry Potter and the Philosopher's Stone, Harry speaks to a snake while visiting the zoo with the Dursleys.
The fact that Harry is a Parselmouth plays a major role in Harry Potter and the Chamber of Secrets, as it is his ability to understand Parseltongue that allows him to hear the Monster in the Chamber when it is prowling through the school, and because it allows him to actually open the Chamber. It also plays a lesser role in Harry Potter and the Half-Blood Prince where it allows him to understand the conversations between members of the Gaunt family as viewed in Dumbledore's Pensieve. It also plays a small role in Harry Potter and the Deathly Hallows where it allows Harry to open the locket Horcrux, understand the false Bathilda Bagshot, and understand Voldemort's instructions to Nagini. Ron imitates the sounds Harry made while opening the locket in order to open the Chamber of Secrets in that book, but that is not really Parseltongue but rather simple parroting.
Greater Picture edit
We learn in Harry Potter and the Deathly Hallows that Harry's ability to speak Parseltongue is actually related to the soul shard that Voldemort lost when trying to kill Harry. That soul shard had attached itself to Harry, and was the source of this ability, as well as the ability to see into Voldemort's mind. The soul shard was destroyed in Harry Potter and the Deathly Hallows, and Harry's ability to speak Parseltongue went with it, according to the author.
The reason that Dumbledore can speak Parseltongue is never made entirely clear, but it is mentioned that he could speak some hundreds of languages, including Mermish and Gobbledegook. Having him understand Parseltongue as well would be relatively simple; apparently the ease of learning a language increases with the number of languages learned, even for Muggles.
While it is not directly associated with the language, we note that Harry is usually unable to tell whether he is speaking or hearing Parseltongue. When addressing the entrance to the Chamber of Secrets, for instance, he only realizes that he had spoken Parseltongue when Ron tells him it wasn’t English. No real explanation for this is given to us, nor is there any explanation for why Harry can tell the difference when visiting the House of Gaunt in Harry Potter and the Half-Blood Prince, but not in the following book in Bathilda Bagshot's house. We suppose it may have something to do with environment; Harry may be more likely to expect Parseltongue from the clutch of odd people in the Gaunt shack than in Bathilda's house. It is possible that this lessened ability to tell the difference came about because the ability was simply dropped on him; if he had to work to learn the language, as we suppose Dumbledore did, he would be much more likely to differentiate. As it is, it is something he has always known, and so seems as easy for him as English. (Even among Muggles, people who, as children, have had somewhat equal exposure to a standard language and a milder one of its dialects will usually switch between them as the occasion requires, and will not instinctively feel a difference between the standard language and their own dialect when hearing or speaking it; though if they set their minds to actually thinking about it, they can make the distinction, as again apparently Harry can.) This inability to distinguish is a minor plot point at two places in Harry Potter and the Chamber of Secrets, and a somewhat larger one a Bathilda's house in Harry Potter and the Deathly Hallows.