Tell me about abstract and concrete class differences?

An abstract class is a rule (an instance of the Rule-Obj-Class class) created to support the definition of rules, including other classes. Such rules can be inherited by subclasses of the abstract class. Rules with a class as a key part (such as properties, activities, flows, models, and so on) can apply to an abstract class.
A class that is not abstract is concrete. Unlike abstract classes, concrete classes can have instances stored in the database. A selection on the Class rule form determines whether a new class is abstract or concrete.