react-router contains all the common components between 
react-router-dom and react-router-native. 

When should you use one over the other? 

If you're on the web then react-router-dom should have 
everything you need as it also exports the common components 
you'll need. 

If you're using React Native, react-router-native should 
have everything you need for the same reason. 

So you'll probably never have to import anything directly 
from react-router.