Cabin types vary depending on the ship and the shipping company.
If you choose "cabin berth" for one passenger, then the passenger will not have his own cabin, but will share a cabin with other passengers of the same sex
If you want an exclusive cabin, then select the "Whole Cabin" seat type.
The difference between an internal cabin and an external one is based on the location on the ship. The exterior has a window while the interior does not.