Abstraction: Hiding implementation details and showing only essential features using abstract classes or interfaces.
Example:
Abstract Class:

abstract class Shape {
 abstract void draw();
}
class Circle extends Shape {
 void draw() { System.out.println("Drawing Circle"); }
}
Shape s = new Circle();
s.draw();

Interface:

interface Animal {
 void sound();
}
class Cat implements Animal {
 public void sound() { System.out.println("Meow"); }
}
Animal a = new Cat();
a.sound();