Flyff (Fly For Fun) is a free-to-play MMORPG. You can buy in-game items from a shop, but no subscription is needed. Flyff was my first MMO, and then I thought it was great. The class systems were easy, the UI was easy, the concepts were easy. After I tried WoW, though, all that changed. xD I would say Flyff is mediocre now, especially since they've horribly changed the UI.
Hearthstone is a branch off of WoW; it's the online version of their trading card game. It's simple and fairly easy to learn (though there is a whole other level of nitty-gritty, but for general play it's not necessary), and what I feel is nice is that you don't need a lot of prior knowledge to the lore behind WoW. I like it, especially when I'm in the mood for a laid-back, non-competitive game.
Really in-depth question to a simple answer, sorry. Lol. xD