How many miles high are satellites?

Can you tell me, how high exactly do satellites orbit the Earth? I'm curious to know if there's a standard altitude for them, or if they vary significantly depending on their purpose. Do they hover just above the atmosphere, or do some reach much higher altitudes? And what factors determine the height at which a satellite orbits? I'm sure it's a complex topic, but I'd appreciate if you could give me a basic understanding of the altitudes satellites occupy.