What is the difference between RER and Metro in Paris?

Related questions

1. What is a RER station in Paris?
2. Is Paris known for pickpockets?
3. How do you pay for Metro in Paris?
4. Is Paris Metro 24 hours?
5. Is Paris dangerous at night?
6. How long is a Paris metro ticket good for?
7. Is Eiffel Tower included in Paris Pass?
8. What time does Metro stop?
9. How much is dinner in Paris?
10. Which metro stations are closed today?

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18. #suburbs
19. #entrance
20. #airports