Savage and civilized are words that are often used when discussing what began as Hegel's self/other concept. He posed the idea that we separate ourselves into two groups, the Self, the known, the civilized, and the Other, the unknown, the savage. This idea is typically used when discussing colonialism, and is thus appropriate when discussing the Tempest, by William Shakespeare. The island in this play turns civilized people savage and visa versa. It is important to determine what characters start off as civilized or savage, and how they change.
As far as Ariel and Caliban are concerned, one way of determining who is civilized, is by who is 'loftier'. "Ariel" means "of the air" and thus has an elevated (civilized) status. Caliban sleeps on the ground (savage).
I think it's safe to say that Caliban is savage because, according to Prospero, he tries to rape Miranda.
You can make the case, as some have, that no one in the story is savage because to do so would assume that one group is superior to another in the story. This raises some serious questions that pertain to colonialism. Montaigne insisted, for example, that the native American indians he read about were far superior to the crooked European cultures he knew of.