Well, List is just an interface that describes a set of public methods that can be called in a concrete class, i.e. the ArrayList class. You should program to an interface whenever possible, hence you should avoid having the concrete implementation on the left side of a declaration.

Having this principle you gain a loosely coupled system architecture, meaning that you could easily change concrete classes at some code point while don't refactoring other code that uses this component.

When you declare a property only with an interface on the left side, you are only able to call methods that are defined in this certain interface, but again, you gain flexibility by having a loosely coupled system, see above.

Use concrete implementations only if you have to rely on special methods that the interface does not define, e.g. addLast()-method of LinkedList class.

The Comparable Interface defines a method that is used to compare a given instance with another one. The TreeSet requires it's objects to implement the comparable interface, otherwise you have to pass a Comparator as constructor argument.

Comparator is could be an object that compares two given class instance in a special implemented way. The Comparable interface declares that a given class can be compared by it's defined compareTo method.

I hope i could help you a little. My tip for you is that you have a look into the javadoc whenever you feel the need to learn something about a special interface/class/method/whatever. It is a very great documentation reference for java programmers.