It depends. If, for example, you already had an earlier call like this:
then, your first instance would refer to the same String object in the String pool as r does. Otherwise, it would create a new String in the String pool. Your second instance creates a new String with the value "abc" regardless of whether a String with that value already exists, and should be avoided.
Not quite. The first statement creates one String, which is created at class loading time. That is the "abc" on the right. Then it applies the name "s" to it. You have one String. This is independent of whether there is a String "r" or not. If you had another reference as John de Michele showed, you would still have one String, but now with two names, "s" and "r".
The second, as John de Michele has told you, creates two objects, "abc" and s which is identical, but a second object. Remember that "abc" is still available for use anywhere else. The second style is, as you have been told, usually best avoided.