What is the difference between Metro and RER in Paris?

Related questions

1. What is the main metro station in Paris?
2. How much is a taxi from CDG to Paris city center?
3. How much does RER cost in Paris?
4. How much is a taxi from Charles de Gaulle to Paris?
5. Is there an app for Paris Metro?
6. How much are metro tickets in Paris?
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2. #network
3. #gaulle
4. #while
5. #regional
6. #serves
7. #metro
8. #never
9. #versailles
10. #disneyland
11. #ensure
12. #stations
13. #paris
14. #served
15. #airports
16. #charles
17. #suburbs
18. #express
19. #nearby
20. #travel