While implementing Comparable you need to modify the class whose instances you want to sort. In Comparator, you will be creating an whole new class, making it reusable.
Comparator gives you the ability to sort objects is different ways and UNLIKE comparable you can use it sort instances of any other class(including the classes you dont have access to). Hence its extensible.
Looking at the code, you should be able to figure out the remaining differences between them.
Try to make comparable and comparator work with TreeSet one at a time.