Interface vs. Abstract Class

Abstract Class	Interface
Cannot instantiate.	Cannot instantiate.
Can only be used to derive.	Can only be used to derive.
Can contain data and/or implementation in the form of concrete methods.	No data, no implementation. (All methods/virtual functions must be abstract/pure.)
In C#: declare both class and abstract methods with abstract keyword.	In C#: declare class with interface keyword. All methods become virtual.
In C++: declare at least one pure virtual function using "= 0" syntax. Can use destructor, too.	In C++: no concept of interface built into C++; interface is a design pattern you must follow.
Use when you want to provide a partial implementation with some functions omitted.	Use when you want to define the abstract qualities of a thing, the methods a class must implement to "be" that thing.