This game is basically a country. A landscape filled with hundreds of quests, thousands of NPCs and endless multi-level dungeons and caves.
It's an RPG, and probably the best action-rpg i've ever played, and I've been gaming for 15 years. I wont go into the story, because you can read up on that elsewhere, or better yet, experience it yourself by playing the game. Even though it's been out for almost 5 years, i still consider it the best looking game on PS3 to date. I've played Fallout: New Vegas and Gran Turismo 5 (recent titles) and Oblivion still looks better. You can seriously get immersed in this game and lose yourself (and sense of time).
It's a level-based game (1 to 100 i believe) and theres an endless combination of character classes you can make, not to mention all the interesting weapons / items in the game to keep you occupied.
I recently jumped back into it and started a new character which I've put about 80 hours into, and he's only level 16... So it will definitely keep you busy for awhile without getting boring. BUY IT!