I myself would explain it as a side scrolling, free roam sand box game.
While you can only view the world from a 2-D perspective, you can move in 4 directions. Left, right, up and down.
You are not limited to the exploring aspect that comes with randomly generated worlds, but also free to destroy and build.
While there certainly is much to do in the way of renovations and exploring, the world is also full of challenges anywhere you might look. You can fight a variety of monsters and boss's at any stage in the game.
A player can feel growth through the in game character as you level up your gear through effort and luck. You can also feel growth as you attempt to fight every creature in the game and are forced to grow and adapt to each creature in order to attain victory.
That is how I would briefly describe it to someone interested in trying it out.