It's a non-linear game with quests you can choose to follow or set aside for later. They've constructed a world for you to explore, and it's very, very pretty. It's an RPG in that there is story behind everything, which you can learn as much or as little about it as you'd like, and your character will gain skills, level up, and acquire "perks" in whatever skills you use. You don't choose a "class" like typical RPG's but rather just get better at whatever it is you do when you play.
The game is as hardcore or casual as you choose to make it, and has near limitless potential for time investment with infinite number of quests and a modding community (if you get it on PC only) that will introduce new content for you to peruse.
If you like stories, exploration, and/or role playing then this game is for you. It's the benchmark for single player rpg/sandbox games. It is "the divorcee maker."
If this doesn't help then tell me what games you're into.