abstract factory (AF) : Defines an interface for creating families of objects without mention there concrete classes.
usage: 1 tablet per day. Oops. That is not for abstract factory. This is: AF encapsulates how objects are created. You don't know what implementation the AF is returning. So it is used when you want to change the objects implementation without affect clients of the object.
factory method (FM): Define an interface for creating an object, but let the subclasses decide which class to instantiate. The Factory method lets a class defer instantiation to subclasses.
usage: the concrete creation for an object O is in subclasses Sb, while the object O is probably used in the super class Sp. You use this pattern when you want to utilized inheritance and polymorphism of OO Design: The method M() that uses the created object O is in super class Sp, while the object O's creation is in sub class Sb. Client C use sub class Sb to do the work (C knows Sb) while the method M() that client is using is in super class Sp. Now, DO you understand?! If you can't, try hard, just like this: