Both interfaces and abstract classes are tools in OOP used to define contracts for other classes, but they serve different purposes.
Abstract classes can include both concrete behavior and abstract declarations, while interfaces typically only declare methods without implementing them (though modern languages sometimes allow default methods in interfaces).
Abstract classes are used when classes share a strong relationship and common functionality, whereas interfaces are used when classes share capabilities but may be otherwise unrelated. A class can inherit from only one abstract class but can implement multiple interfaces, making interfaces a flexible way to achieve multiple inheritance.