What is implementation and interface inheritance?

When a class (type) is derived from another class(type) such that it inherits all the members of the base type it is Implementation Inheritance.

When a type (class or a struct) inherits only the signatures of the functions from another type it is Interface Inheritance.

In general Classes can be derived from another class, hence support Implementation inheritance. At the same time Classes can also be derived from one or more interfaces. Hence they support Interface inheritance.