Oberon, King of the Faeries, Titania, Queen of the Faeries, and Puck are the most important faeries in the script. The faeries exist in a spiritual realm and are not seen by mortals. Faeries can, however, manipulate the mortal realm.
Puck is Oberon's lieutenant, or high-ranking servant. (Puck's name is Robin Goodfellow.)
Queen Titania has a "changeling boy." This child was born of a mortal friend of Titania's. She has passed away, and Titania is raising the boy. Oberon is extremely jealous. Not only is Titania no longer paying attention to him, but Oberon also wants the child for himself to be his servant.
Oberon plans revenge. He summons Puck, and tells him to find a magical flower. When the juice of the flower is dabbed on sleeping eyes, the victim falls in love with the first thing he or she sees upon waking. Oberon instructs Puck to use the flower on Titania, and to ensure that she sees something vile when she wakes up in the morning.
In addition to obeying Oberon's commands, Puck has some fun on his own.