What is the difference between an interface and abstract class?

Question

What exactly is the difference between an interface and abstract class?

Answer 1

I'd like to add one more difference which makes sense. For example, you have a framework with thousands of lines of code. Now if you want to add a new feature throughout the code using a method enhanceUI(), then it's better to add that method in abstract class rather in interface. Because, if you add this method in an interface then you should implement it in all the implemented class but it's not the case if you add the method in abstract class.