You can book a specific cabin, or you can book a guarantee. If you book a specific cabin, that is the cabin you will get (99.9% of the time). If you book a guarantee, you save a little money, you have no control over what cabin you will get, but you could end up in a better cabin than you are paying for (the upgrade would be free). I believe that if you book a guarantee with three people, you will get a cabin that is set up to sleep three.
In general the least expensive cabin is an inside cabin. Then an oceanview cabin. Then a balcony cabin and finally a suite. The lower (physically) you are in the ship, the less expensive the cabin is. So, an inside cabin on the top deck might cost more than an oceanview cabin on the bottom deck. An oceanview cabin on the top deck might cost more than the lowest balcony (balconies are never close to sea level).