Abstract class: A class declared as an abstract class cannot be instantiated, but it can be subclassed.
You can declare a class as abstract when a class is merely a representation of commonalities among its subclasses in which case it does not make sense to instantiate objects of that class.
The Animal class that exists as a generalization of its subclasses Cat, Dog, Horse, Tiger etc. can be declared as abstract because it does not make sense to instantiate an Animal object.
Abstract method: An abstract method is a method signature without a method implementation.
The move method of the Animal class is likely to be an abstract method as it is not possible to implement a move method at the Animal class level to fit all subclasses because each animal type can move in a different way.
A class that has an abstract method becomes an abstract class because the class definition is incomplete (due to the missing method body) and it is not possible to create objects using an incomplete class definition.