In Python, an abstract class is a class that contains one or more abstract methods. Abstract methods are methods that are declared but do not have an implementation. These methods must be implemented in the subclass. An abstract class can be used as a blueprint for creating other classes.

Interfaces, on the other hand, are not implemented using classes, but rather as a set of methods or functions. An interface in Python is simply a collection of methods that a class can implement. They define a set of rules, but do not provide any implementation details.

Abstract classes and interfaces serve similar purposes - they can be used to define a common set of rules that a group of classes must follow. However, abstract classes provide a partial implementation of the class, allowing subclasses to inherit that implementation. Interfaces, on the other hand, only provide a set of rules that the implementing classes must follow, with no implementation details included.

One common use case for abstract classes and interfaces in Python is to create plugin systems. For example, you might have an application that can load plugins at runtime, each of which must implement a certain interface or be a subclass of a certain abstract class. This allows you to create a modular, extensible application that can easily add new functionality without having to modify the core code.