Typically, gameplay centers around one or more avatars, with quantized characteristics that evolve over the course of the game, and take the place of the gamer's own skill in determining the outcome of the actions taken by the player. These attributes are traditionally displayed to the player on a status screen as a numeric value, instead of a simpler abstract graphical representation, such as the bars and meters often favored by video games in general. Another common element in RPGs is a well-developed fictional setting.
That is the definition of a RPG. People will argue one way or another, but they are all wrong. People think Zelda is a RPG, it's a action adventure with RPG elements. Most games now a days have RPG elements, basically if you can't understand the definition posted it means "If a level system exists which is crucial towards game play, and your decision influences the characters in any way it is an RPG".
There is so many sub-genres of RPGs it's ridiculous now. Take Folklore, it's an action adventure RPG. Why is Zelda only action adventure when folklore is an action adventure RPG? Because folklore has a leveling system, sure it's leveling system isn't that crucial but it has one. COD4 can be seen to have RPG elements because you can build a character online. God of War has RPG elements because you can upgrade weapons and unlike other games choose which to, to determine how you play. Even zelda, which is widely argued to be a RPG is not, but it has RPG elements, such as you determine how the character progressed, level up the health, the weaponry, etc.
Fable II however is NOT just a RPG. People who say it is, are morons. A RPG was first based off a game emulating the table top RPGs, such as dungeons and dragons. In that respect, Fable II is an action adventure RPG, because of the combat. Turn-based I hate to say it is still the closest thing you can classify as just a RPG. Not to say fable 2 isn't an RPG, it's just a sub-genre.
Today there is so many, FPS-RPG, Action-RPG, Action Adventure-RPG, RTS-RPG, RPG(See turn based and classic) etc etc.
Examples.
FPS-RPG - Resistance 2, online co-op.
Action RPG - Phantasy Star, Star Ocean(maybe)
Action Adventure RPG - Folklore, Fable 2