Ships' most important property is the ability to travel between the stars. Indeed it is this property, made possible by their warp and jumpship generators, that makes galactic empires at all feasible. There are many kinds of ships but most can be grouped into the following types:
Fighters are inexpensive and thus very abundant. They make excellent protection for large fleets and in addition are able to operate in a planet's atmosphere (other ships, although able to enter the atmosphere and even land, are very vulnerable during re-entry and never attempt to do so during combat). Fighters can be created by the most basic space faring technology.
Transports are the weakest of ships from a military standpoint. They are unarmed and unarmored and thus need the protection of other ships. Their great size, however, makes them capable of carrying large amounts of material or troops across the empire. Most transports are able to carry from two to three megatons of material (depending on the type of material).
Jumpships are small, lightly armored ships equipped with jump generators. Like hunter-killers and jumptransports, these ships are able to travel up to ten sectors per year. Although not as heavily armed as penetrators or hunter-killers, jumpships are very effective combat ships, especially against fighters. Because of this and their speed, they are ideal components of a rapid response or hit-and-run strategy.
Although smaller and able to carry only about a fifth of a transport's hold, jumptransports are armored and thus make ideal troop carriers. As their name implies, jumptransports are equipped to travel in jumpspace, enabling them to move up to ten times faster than conventional transports.
Hunter-killers combine the speed and agility of jumpships with the sophisticated counter-detection ability of penetrators. Unlike penetrators, however, hunter-killers are also nearly undetectable at short ranges. Fleets of hunter-killers remain unseen even when in the same sector as an enemy world. In combat, they can advance through enemy lines without being seen and then attack targets at inner orbits. Although they have respectable fire-power, they are not heavily armored and are easily disabled.
Although penetrators do not posses a jumpdrive, their warp generator is modified to travel twice as fast as normal warp ships (two sectors per year). In addition, they are equipped with sophisticated long-range sensor jammers which make them nearly invisible on an enemy's scan. Although ineffective against short-range sensors, the electronic jamming allows whole fleets of penetrators to enter deep into an empire's territory undetected.
Large and very expensive, starships are the most powerful components of a fleet. Although they possess neither the jump capability of jumpships nor the stealth technology of penetrators, the armor and firepower of starships make them immune to anything but the most powerful weapons.