There are three broad categories of vessel:
(1) bulk: liquid/wet/tankers and dry,
(2) intermodal: containers, and
(3) specialized (everything else)
The bulkers carry cargo that is not unitized and tends to be in a free flowing state either liquid/wet (tankers) or dry, whereas intermodal ships are mainly container carriers.
Size in bulk carriers is primarily expressed in Metric Tons Deadweight (MT DWT) which in effect is the cargo carrying capacity of the ship including bunker fuel and constants. Their Length Over All (LOA) is another principal characteristic as well as Draft (max depth of immersed hull).
Size for Containers is primarily expressed in number of TEUs (boxes of Twenty Foot Equivalent Unit) that they can carry.
Smaller sized bulkers (Panamax and below) invariably are equipped with their own gear (deck cranes) and thus can be self-loading/unloading. After that, there are several other specialized types of vessel such as Livestock Carriers, Refrigerated Ships (Refers), Car Carriers, Natural or Petroleum Gas Carriers, Deep Sea Drilling Ships, Barges, Cement Carriers, Ice Breakers, Offshore Supply Vessels (support to Oil /Rigs Platforms), Fishing Ships, Dredgers, Passenger Ferries and Cruise Ships, and Ro-Ros (Roll on- Roll off ferries).