Well, I think ethanol is generally on the right track, he's over simplified both themes quite a bit. Most RPG's are strategy oriented which is like the Final Fantasy, Dragonquest, or Baldur's gate (PC version) series'. With action RPG's there is a much wider lattitude in terms of what it can mean. Basically an RPG refers to any game where you can "level up" your character, or slowly improve him/her over the course of the game. This is like Diablo, Baldur's gate: DA, Secret of Mana, Dark Cloud, or even the Zelda series.
I'd compare Baldur's gate: Dark alliance to an advanced, highly polished Gauntlet type game. Add in the RPG elements, tons of armor and weaponry and you have an excellent mix.
It's not as bad a button masher as diablo, especially with the different types special attacks (Actually the dwarf is a button masher character, the other 2(3) require some finesse)
You won't be thinking too much about that though, as you trek through dungeons, ice caves, the countryside, swamps and who knows what else. You'll just be hoping for the next high powered sword which is just around the next bend....