I'm not at all knowledgeable in the subject, but from reading this it appears to me that they are quite different.
"Plan Cache" is used to cache the compiled plans in case of reuse opportunities.
Whereas "Procedure Cache" is used to store the execution plans, as per the original question.
Nope. Compiled plans are execution plans. The area of memory where plans are stored used to be called the procedure cache, it's now called the plan cache. They are not distinct areas of memory storing different things, they're two names for the same thing.
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