To creep = to move slowly, quietly and carefully, usually in order to avoid being noticed - it can be on hands and knees, or on foot.
To crawl = to move slowly or with difficulty, especially (of a person) on hands and knees.
The essential nature of creeping is the desire to avoid being seen, whereas crawling can merely indicate very slow movement.
So a cat will creep up on its prey, while a spider can crawl slowly up a wall.
Snakes don't crawl since they have no external limbs with which to crawl.
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