Dahl and Nygaard are considered the fathers of Object Oriented Programming (OOP) and invented the concepts of classes, subclasses, inheritance, etc., in their programming language Simula (which is based on ALGOL-60).
As the name suggests Simula is primarily used in simulation. Some professors still choose the language to teach introductory programming courses. Simula greatly influenced Bjarne Stroustrup who wanted to bring together benefits of OOP with the speed of lower level languages as part of C++. Simula therefore prepared the road for other programming languages such as C++ and Java.