Try to call a method on both of them and you'll see the difference
Basically a reference to an empty string points to an object in the heap so you can call methods on it. But a reference pointing to null has no object to point in the heap and thus you'll get a NullPointerException...
Seeing above replies now you know that in null case there is no object created and currently variable is pointing to nothing, while in later case, there is an empty string object created and variable is pointing to that. Now you take println in both cases and see what it prints.
Think nothing of it; we all make that sort of mistake.
And there is another spelling error; it should read null not NULL in the thread title. Remember Java is case-sensitive and you can get all sorts of difficult-to-find errors if you put a capitaL letter in the wrong place.