Short answers, pulled from the story.
A passenger ship is a merchant vessel whose primary function is to carry passengers at sea. Cargo ships that offer limited berths for up to twelve passengers do not qualify, because in those cases transporting freight is the principal purpose.
Knut Kloster, the director of Norwegian Caribbean Lines, purchased one of the largest surviving ocean liners and converted her into a cruise ship, renaming her the SS Norway. Her success demonstrated a market for large cruise ships and triggered successive orders for ever-larger vessels.
The Oasis of the Seas was completed in 2009 as the first of the over 225,000 GT cruise ships. Her displacement reached 100,000 tons, well under half her gross tonnage figure.
Gross register tonnage, the older measure, counted only certain enclosed spaces in units of 100 cubic feet. Gross tonnage, adopted in 1982, uses a mathematical formula covering the total moulded volume of all enclosed spaces and is used to set manning rules, safety requirements, registration fees, and port dues.
Passenger ships must conduct musters of passengers within 24 hours of embarkation. They must also be capable of completing full abandonment of the vessel within 30 minutes of the abandon-ship signal being given.
The International Ice Patrol was formed in 1914 in direct response to the sinking of Titanic, to address the danger of iceberg collisions in North Atlantic shipping lanes.