Puck is also known by the name Robin Goodfellow.
Puck is a sprite or Hobgoblin, a fairy from British folklore. He is a trickster and magic user, and serves Oberon, the king of the fairies.
When Puck meets the fairy, she knows immediately who he is.
Either I mistake your shape and making quite,
Or else you are that shrewd and knavish sprite
Call'd Robin Goodfellow. (Act 2, Scene 1)
Puck acknowledges that he is a prankster, and that he is Puck/Robin Goodfellow. He describes himself as “that merry wanderer of the night.” He “jests” with Oberon, and serves Titania. In his spare time he has some fun with human beings, playing pranks on them like pretending to be a stool and then pulling it out so someone falls down.
Puck’s role in the play is actually pretty important. He anoints the wrong Athenian youth, and gets the lovers all mixed up. He replaces Bottom’s head with that of a donkey, causing the craftsmen to run away. He then makes sure Titania sees him first when she wakes up, so she falls in love with a mortal wearing a donkey head. Although Oberon is annoyed at his mistakes, Puck generally does intent to follow orders, and he usually gets them right.