Movies are rated on a system devised by my movie-watching cabal based on the simple question: How much worth your time and/or money is seeing this film?
There are six levels of ratings. They're easy to remember, and they even proceed in alphabetical order:
A (Advance Showing): Some films are such must-sees that they're worth paying extra and going out of your way to catch—as you might do for an advance (a.k.a. special sneak preview) showing.
F (Full Price): A film rated Full is worth seeing on its intital run in the theaters, even though you'll have to pay the full ticket price. It'll be worth it.
M (Matinee): Matinee movies are worth seeing in the theaters, but only if you can get a discount on the ticket price. They're good—usually a lot of fun—but probably not worth seeing more than once.
R (Rental): Rental flicks have redeeming qualities, but they're ones you definitely won't mind catching on video. The screen may be small, but you don't want to pay even a matinee ticket price for this kind of film.
TV: A movie that gets a TV rating isn't worth spending any money on. If it comes on TV, you probably wouldn't mind spending a few hours to catch it, but otherwise you can avoid it with a clear conscience.
W (Worthless): This bottom category is exactly what it says. A Worthless film is one that you should skip even if it comes on TV and you have nothing better to do.