Yes there is a difference between RPG's and adventures. In pure adventures, combat, level advance of your character, resting and moving from city to city etc do not exist ( except Sierra's "Hero quest" ). You only solve riddles and by solving them you advance the story of the game - much like a interactive movie ( well, sort of ). Diablo is truly a sacred game, but its not a adventure game
the example that Dany252 gave as a adventure was almost like Diablo, and thats why i stated that.