What is the difference between abstract classes and interfaces?

Abstract class:

• Can contain abstract (without implementation) and concrete (with implementation) methods.
• Can contain fields (instance variables).
• Can have constructors.
• Allows single inheritance (one class can inherit from one abstract class).
• Used to define a base structure and common behavior for related classes.

Interface:

• Can contain only abstract methods (some languages, like Java from version 8, allow default and static methods with implementation).
• Cannot contain instance fields (only constants).
• Cannot have constructors.
• Allows multiple implementation (one class can implement multiple interfaces).
• Used to define a contract that classes are obliged to implement.