You've basically answered your own question - RPGs can be multiplayer, sure, but an MMO is that but on a larger scale, hence the "massively" bit. So all MMORPGS are RPGS, yes, but not all RPGs are MMORPGS. It's a matter of scale. For something like Torchlight, where it's a multiplayer RPG, it's not an MMO, because the amount of other players you'll interact with is bounded - likely you and a few friends. With WoW, Star Trek Online, etc, it's a much bigger scale. Hundreds of players can be in the same location at the same time, interacting with each other.
The other aspect that often differentiates the two is the persistence - MMOs tend to be always on - it doesn't matter if you're active in the game or not, the game is still going, running on some servers somewhere all the time, waiting for you to log in and interact. Whereas for regular multiplayer games, there's not this persistence.