Understanding Engine Vacuum and MAP - What is Vacuum? Why engines have vacuum & why it's important

Gasoline engines with throttles develop vacuum in the intake manifold. That vacuum provides information about how the engine is running and can be used to diagnose problems.
So what is vacuum, why does an engine have vacuum and how is it measured? What causes it to be high or low, what is normal? What is MAP and how does it relate to vacuum?

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