What you're describing is an interface. An interface is a contract that ensures you can do something, but doesn't care how.

Abstract classes can have abstract functions. When you do this, you're basically just creating a base class with an interface on top; you're saying "all my children must implement this interface of mine" without having to actually make a separate interface.

Abstract classes also offer additional functionality though, such as the ability to define properties, and default implementations of methods. You can even utilize the base class implementation of the method in your child class, in order to perform extra steps or format your input before you do whatever it is you were doing in the first place.

So, an interface is a contract that allows you to call a method, without having to know the specific class or implementation.

Inheritance is more like "it does everything that X does, but it also does Y and Z." If you're ever finding yourself writing an abstract class with purely abstract methods, you probably want to write an interface instead. That way, you get all the same functionality, but it's more loosely coupled

Epecially when you think in "real" OOP terms:

Abstract classes are "child is a parent", fx "duck is a bird". Bird describes all the traits that all birds have in common. But not all birds fly, so flight must come from an interface. This interface can be passed around to any number of objects, and they're not as tightly coupled because unlike an abstract class, an interface doesnt imply that "duck is a flight". The interface is just something we know the duck can do.

As you can probably tell, I work with OOP on a daily basis and have for years. There are a lot of valid criticisms of the OOP philosophy, and I have heard a lot of good points for the record. I am just educating on the OOP principles because you said you were interested and to clear up any misconceptions.