OOP concept: A class is a definition for a particular kind of object. It may define the data and methods that will be included in an actual instance of the object.
SIDL keyword: class is a SIDL keyword. In SIDL a class definition only defines methods. Methods may be static or instance methods. (They are instance methods by default.) If any instance method in a class is declared abstract, the class cannot be instantiated as an object, and is called an abstract class. Otherwise, it can be instantiated and is called a concrete class.