Comparison of lists with wildcard types: extends, super, and specific type

It is necessary to understand the differences between collections declared using wildcard types.

• How do upList, downList, and exactList differ?
• What elements can be added to each of these collections?
• What elements can be retrieved from each collection?

class A {}
class B extends A {}
class C extends B {}
List<? extends B> upList = ...
List<? super B> downList = ...
List<B> exactList = ...