There are several key differences between an abstract class and an interface. First, an abstract class can contain both abstract and non-abstract methods, while an interface can only contain abstract methods. Second, an abstract class can have instance variables, while an interface cannot. Third, an abstract class can have constructors, while an interface cannot. Finally, a subclass of an abstract class can only inherit from one superclass, but a class can implement multiple interfaces. Consequently, each type of structure has its own advantages and disadvantages that must be considered when designing a new system.