The keyword abstract has two uses: a class modifier and a method modifier.

Description #1 - Class Modifier

As a class modifier, it specifies that the class is abstract. An abstract class can not be instantiated. It exists only to be subclassed by other classes.

Description #2 - Method Modifier

As a method modifier, it specifies that the method is abstract. An abstract method defines a method signature but not a method implementation. Implementation is deferred to a subclass.

Example

abstract public class Shape{
  abstract public double getArea();
}