Tonnage refers to the volume and there are a few types of tonnage that you will come across frequently.
Gross Tonnage is indicative of the total volume of the of a ship and may be often used in reference to the size of the ship.
Net tonnage is indicative of the of the cargo and passenger spaces in a ship which produces the revenue.
Lightship weight/light weight: Is the weight of the hull steel, machinery and outfit.
Dead Weight: The carrying capacity of the ship namely the total weight of cargo + fuel + fresh water + ballast + Crew & effects.
It is also = Load displacement upto the minimum permitted freeboard – lightweight
The weight of the cargo that can be carried is called as Cargo Deadweight.