Parseltongue

Parseltongue is the language of snakes; to a human who cannot speak it, it sounds like hissing without taking a breath (GF1). A speaker of Parseltongue is referred to as a Parselmouth (CS11).

The ability to speak Parseltongue is extremely rare, and is something for which Salazar Slytherin was famous (hence his nickname of “Serpent-tongue”, and the choice of a serpent as the symbol of Slytherin House). Tom Riddle inherited this ability from Slytherin, and used it to control the basilisk from the Chamber of Secrets. Harry acquired the ability to speak Parseltongue from Voldemort during Voldemort’s attack on the Potters. Harry used the ability unwittingly to speak to a boa constrictor in the zoo (PS2) and to a snake conjured by Draco Malfoy at the Duelling Club (CS11).

• Thanks to Parseltongue, Harry could hear the reptilian Basilisk in the walls of Hogwarts before it petrified victims (CS8).
•  Harry was able to "speak" to Voldemort's horcrux inside Salazar Slytherin's locket using Parseltongue (DH19)
• Unfortunately, Voldemort figured out that Harry was a Parselmouth like himself and used it against him in Godric's Hollow. The Dark Lord turned the dead body of Bathilda Bagshot into an Inferius, placing his snake Nagini inside. Harry "heard" Bathilda telling him to follow her, but it was actually the snake speaking in Parseltongue, and when they were upstairs in Bathilda's house, the snake attacked Harry (DH17).
• Most wizards in history who could speak Parseltongue were descendents of Slytherin from the House of Gaunt, and only they knew how to open the Chamber of Secrets (Pm).
• Isolt Sayre, niece of Gormlaith Gaunt could understand Parseltongue but not speak it (Pm). That is why she could understand what the Horned Serpent in the creek near Ilvermorny was telling her when it warned of impending doom if it was not made "part of the family." The Serpent allowed Isolt to take part of one of its horns to use for the wands of her adopted sons, Chadwick and Webster. When Gormlaith Gaunt attacked Ilvermorny, using Parseltongue to put Isolt's Basilisk wand to sleep, the language instead awakened the Horned Serpent cores and they sounded a "low musical note" which alerted the boys (Pm).
• It is believed that Isolt's daughter Rionach could speak Parseltongue, but she never married in the mistaken belief that she was the last descendant of the Slytherin line. She was not aware that the Gaunt line continued in England, eventually producing Tom Marvolo Riddle (Pm).
• Nagini used parseltongue when she struck at Skender to facilitate her escape from the Circus Arcana (CG).

• J.K. Rowling confirmed during a 2007 web chat that Dumbledore understood Parseltongue, since he seemed to understand Morfin Gaunt as well as Harry when when viewing memories in the Pensieve (HBP10): "Dumbledore understood Mermish, Gobbledegook and Parseltongue. The man was brilliant."(BLC) Source: Accio Quote

• When Ron opened the Chamber of Secrets during the Battle of Hogwarts, he was merely copying phonetically what Harry said years before, and it was basically just the one word command to "Open" (JKR).
  

  I don’t see it really as a language you can learn. So few people speak it that who would teach you? This is a weird ability passed down through the Slytherin blood line. However Ron was with Harry when he said one word in Parseltongue, which I do not know so I cannot duplicate for you, but he heard him say “Open,” and he was able to reproduce the sound. So it was one word. Whether he could learn to speak to snakes properly is a separate issue. I don’t think he could. But he knew enough, he was smart enough, to duplicate one necessary sound. (OBT/CH) - J.K. Rowling Source: Transcript
  

• The author said when Voldemort was destroyed, Harry lost the ability to speak Parseltongue "and is very glad to do so" (BLC). Source: Accio Quote