lkjjlkm 9 (or whatever your friggin' name is lol) has it pretty on the money. To me there are two rpgs. Action rpgs and true rpgs. Action rpgs are real-time and true rpgs are turn-based. In the end though, I haven't nailed it down to a science, but I've thought about it a lot though. If it's turn-based, it is definitely an rpg (with the exception of strategy games, which can be strat-rpgs, or just plain strategy). The difficulty comes when you're nailing down whether an action-rpg is actually an action or adventure game. At that point, it comes down to how far you can customize your character. Then how much strategizing is involved. This is critical. If you find that the majority of strategic decisions MUST be made in split-seconds, then it could be an adventure game. If you have lots of time to decide beforehand, it's probably action-rpg. But it also depends on the storyline (for the most part, having to do with how long it is). If it's huge and engrossing then it's probably an action-rpg. Examples:

Zelda- an adventure game. Why? Nearly no storyline, Little strategic decisions before battle, and nearly-no customization (pretty clear cut)

PE- VERY CLOSE to being an rpg. And it could be called one with a little tweak in my definition. But IMO the storyline is not long enough and world-encompassing. As well, MANY decisions must be made at that moment. (hard to define)