Ships
Ships define the capacity and strength of your fleets. You can transfer ships between fleets, or from D-Ships to fleets or back again. 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 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.
In order to capture an opponent's fleet, you must fire at and destroy all of the ships on it, and also be the only player with fleets remaining at that same world.