Role of spirits in The Temptest by Shakespeare

Prospero has acquired the ability through magic to call the spirits at will, therefore they appear when he calls. He gained this ability while in exile, magic and sorcery were intriguing.

Ariel (a spirit) serves Prospero because he was responsible for releasing he/she (the part has been played by both males and females) from bondage.

A description of other spirits in the play can be found right here at gradesaver;

"Figures that appear in Prospero's betrothal masque for Miranda and Ferdinand in Act IV. Iris, Ceres, and Juno are goddesses from Roman myth; Iris is goddess of the rainbow, Ceres presides over agriculture, and Juno is queen of the gods. The parts are all performed by spirits, with Ariel performing the part of Ceres."

The link for the above excerpt is provided below;

http://www.gradesaver.com/the-tempest/study-guide/character-list/