Ships
Ships define the capacity and strength of your fleets. You can build ships with your industry and metal. You must have at least one ship on each fleet key in order to keep ownership of it, but there is no upper limit to the number of ships you may put on any one fleet.
The ships define carrying capacity: each ship can carry one unit of metal (unless you are a Trader, in which case each ship can carry two units of metal).
The ships also define military power: each ship on a fleet can fire one shot at your enemies, and it takes two shots to destroy each enemy ship. Thus, the more ships you have, the more firepower you have; and the more firepower it will take to destroy your fleet.
Ships are not permanently associated with fleets--you can transfer ships back and forth between fleets, I-Ships, and P-Ships at will.