This is an essential part of who Caliban is. Caliban symbolises the part of human nature that we as humans have learnt to suppress. The psychologist Sigmund Freud described the id' that it is in all of us, and although Freud was hundreds of years after Shakespeare, we can see a lot of this id' in Caliban. It is the savage, wild, side of us all that is our most simple needs personified. Freud believed that as we grow up, we learn to control this dark side of human nature Caliban is this dark side. Prospero tried at first to control him using kindness: "Thou strok'st me and made much of me,"(I.ii.333) but that didn't work so now Caliban is "Deservedly confined to this rock."(I.ii.361) Just as one has to confine the id to the darker recesses of the mind. If Caliban represents everyone's dark, violent part of himself or herself, this contrasts to the spirit Ariel, who is imagination, and creativity. Ariel can take many different forms, he is magical and mysterious, whereas Caliban is ugly, trapped in one shape, and very basic.
Caliban is also very symbolic of subservience, and represents the natural hierarchy of life. Prospero is very learned, and when he was marooned on the island, all he had was books and his daughter. Caliban, however, is only useful for fetching wood. This is shown through the fact that Caliban's language is very simple. He describes the sun and the moon as the "bigger light and the less that burn by day and night." (I.ii.335-6) He also has a tendency to describe things in lists, "toads, beetles, bats..." whereas Prospero's language is a lot more sophisticated: "That beasts shall tremble at thy din." (I.ii.371) Language represents power, and Caliban is evidently lower on the scale of power. He stands for all those people, in every age, who are suited to the simpler jobs in life, and who let everyone else get on with politics and intellectualism.
One other reading of what Caliban could represent is natives in a time of colonialism. He is the aborigine of the island, and Prospero and Miranda are colonisers. Shakespeare was living in a time when colonialism was beginning, and the British were starting the first colonies in the Americas. The natives of these places were seen as savages, just as Caliban is described as a "villain." The colonisers considered themselves to be superior to those that they were conquering, just as Prospero is a superior to the bestial Caliban. Even the name evokes the traditional image of a native: a cannibal. His savagery is very similar to what the common perception of a native was at the time the Tempest was written. Some even believe that Shakespeare originally intended this character's name to be cannibal.
Caliban could even represent evil itself. Prospero can be compared with God, a ruler of a world, where he is a first vengeful to his enemies, but ultimately forgives them. If Prospero is a God character, then Caliban is evil. Later in the play, he tries to usurp the rule of Prospero, turning certain members from the ship against him. He also is like evil in that Prospero gave him a chance and treated him nicely, until he fell from grace, committed a sin, just like Adam in the garden of Eden, just like humanity today which is constantly committing acts of evil. Caliban went against the rule of Prospero, just as the Devil went against God.