When you hear the word "metamorphosis" your first thought is probably "Aww, a lil' caterpillar turning into a majestic butterfly!"
Not quite what Kafka had in mind.
The conventional translation of Kafka's Die Verwandlung is The Metamorphosis, but the German original actually has a more run-of-the-mill sense of "transformation."
The English word "metamorphosis" is more formal and fancies up the German original by linking Kafka's tale to one of the great works of classical antiquity, Ovid's Metamorphoses, a series of stories describing mythical characters who are punished for sexual misbehavior by being turned into plants and animals.
Interestingly, Kafka's title doesn't specify who or what is being transformed. While Gregor's transformation into a bug is certainly front and center, the generic title invites the reader to consider all of the other ways that Gregor and the other characters are transformed in the course of the story.