Corner bath tubs are semi-freestanding. They don't require a raised platform or dais require custom installation. Theses tend to be the largest tubs marketed.
Drop-In bath tubs are custom units that must be supported from underneath since they do not have an apron. These units are perfect if you have a unique bathroom because they can be built on a raised platform, dais, island, and can even be sunken into the floor.
Tile Flange bath tubs are designed to prevent water damage to your tub. They are drop-in tubs that are supported from underneath but with a gap between the tub lip and the wall the tub sits up against. Calk is then used to seal this gap thus preventing the damaging effects of water on your drywall.
Freestanding bath tubs are very common. Widespread for their ease of installation, they can be put into any home with proper plumbing.