Masterpiece = Pretty self-explanatory. One of the best of the best, and/or a personal favorite of mine.
**** = Excellent. Doesn't quite make it to "Masterpiece" status for one reason or another, but it's a great film definitely worth seeing.
*** = Good. A film worth watching, but has some flaws that prevents it from achieving greatness.
** = Mediocre. I don't recommend it, but it has some redeeming qualities.
* = Awful. Absolutely terrible, not worth the time or the effort.
X = The Worst. About as bad as a film can get.