Short answers, pulled from the story.
Gríma son of Gálmód functions as a traitor and spy who serves King Théoden of Rohan while secretly aiding Saruman. He acts as an archetypal sycophant described by psychologists Deborah and Mark Parker as a flatterer, liar, and manipulator.
The name Gríma derives from the Old English or Icelandic word meaning mask, helmet, or spectre. In Old English wyrm means serpent, snake, or dragon, and Gandalf repeatedly compares him to a snake.
Gríma killed Saruman by slitting his throat and was in turn shot by Hobbit archers during the Scouring of the Shire episode. This final confrontation ends Gríma's journey from advisor to traitor to murderer before meeting his end at the hands of those he once tried to harm.
Saruman had promised him Éowyn, the king's niece, as a reward for his services. Critics note that his behavior exemplifies lechery, one of the seven deadly sins, and he desired her to take her as his own woman.
Wormtongue was voiced by Michael Deacon in Ralph Bakshi's 1978 animated film adaptation of The Lord of the Rings. Peter Jackson's live-action version featured Brad Dourif who shaved off his eyebrows to create an unnerving presence.