Metamorphosis is a process by which an animal physically develops as they grow from a juvenile to an adult.

Animals that under go metamorphosis include insects, amphibians and fish, and usually, but not always, it is accompanied by a change in habitat or behavior.

In insects, metamorphosis can be complete or incomplete:

Complete metamorphosis involves a total change in shape during a resting stage called a pupa.

Incomplete metamorphosis involves a series of less drastic changes which occur each time the animal moults.

Also see:
Larva
Nymph
Prepupa
Pupa
Chrysalis
Cocoon