Battleship
Battleships are truly massive starships, equipped with enormous numbers of weapons and potent Void Shields, and usually serve as the flagship for an Imperial Lord Admiral, though this is not always the case. Although very powerful, Battleships are slow and cumbersome to manoeuvre. The Imperial Navy employs three main classes of Battleship: the Emperor (which can carry an immense amount of Attack Craft in addition to its normal weaponry),the Victory (known for its heavy Lance batteries) and the Retribution (which is noted for its powerful broadside firepower and its large number of torpedo tubes). The Battleships of Battlefleet Armageddon are based on the older Apocalypse and Oberon-classes. Other Segmentae's battlefleets may rely on yet other starship classes, but the Emperor and more recent Retribution-class vessels are by far the most common Battleship classes in the Imperial Navy. Imperial Battleships can have crews of anywhere between 25,000 to 3,000,000 men or more depending on the source consulted, including large numbers of Imperial Navy armsmen (marines in modern parlance) to defend against enemy boarding assaults. Battleships can be up to 8 kilometres from prow to stern and displace billions of tons. Because they represent such a vast expenditure of resources to construct and require a fairly advanced technical base to maintain, Battleships are typically constructed only in the largest Imperial shipyards in orbit of the major Adeptus Mechanicus Forge Worlds. These vessels are precious assets of the Imperium and are carefully husbanded, usually employed in only the larger fleet formations and the most critical engagements.
For the Imperial Navy officer class (admirals, captains, commanders, first lieutenants, etc.) on board, an entire section of a large Battleship's command deck may be divided into huge, luxurious staterooms, with vast bedrooms, offices and wardrobes, bathrooms and toilets mirroring those of an Imperial Planetary Governor's palace, and even harems of concubines and handmaidens. Lower-class bridge officers and chief petty officers would have fairly large, moderately luxurious quarters, and the armsmen, mid-class crewmen and petty officers would all have decent accommodation, generally consisting of a sleeping area, chemical toilet and storage locker. However, the bottom-class enlisted conscripts and slave ratings would have little more than a flea-infested sleeping pallet and a single toilet shared between ten ratings.
Grand Cruiser
Grand Cruisers are smaller than Battleships, yet larger than Cruisers. These vessels are usually much older in design and do not incorporate many of the features that are typical in the more current classes of Imperial Navy vessels, like the armoured prow, and are not quite compatible with current Imperial Navy tactical doctrine. As such, many are retired from active duty, but are still used by the Imperial Navy's reserve fleets. The Avenger-class, with its powerful broadside firepower, is one such example of a Grand Cruiser. There are also some modernised versions of Grand Cruisers in active service, but since these are much larger and more heavily-armed than their predecessors, they are more often classed at present as Battleships. These kinds of vessels are usually purpose-built or modified from Battlecruiser hulls and are not commonly encountered in the Imperial Navy.
Cruiser
Cruisers make up the majority of an Imperial Navy fleet. Though not as powerful as a Battleship, Cruisers are much faster and can still deliver a deadly blow. There are multiple classes of Cruiser, most based on the same general hull design but incorporating different combinations of broadside batteries, Lance turrets and Attack Craft hangars. Examples include the all-round, ubiquitous Lunar-class, the Gothic-class with its powerful Lances and the Dictator-class Attack Craft carrier. Cruisers can carry a crew complement of anywhere between 10,000 and 1,000,000 crewmen (including Imperial Navy armsmen and military police squads), depending on the sources consulted. While Cruisers are still particularly technologically complex, it is not uncommon for them to be constructed on smaller Forge Worlds or any Civilised World that has an orbital shipyard suitable for constructing vessels of their large size. Of particular note was the Lunar-class Cruiser Lord Daros, constructed in orbit above the Feral World of Unloth in 11 Terran years by relying on the forced labour of most of that planet’s population for materials.
A note on Imperial warship classes: while naval warships can be defined along a fairly limited number of classes based on weapon configurations approved by the Adeptus Mechanicus, the physical form these vessels take varies widely throughout the Imperium. For example, a Lunar-class Cruiser constructed above Cypra Mundi in the Segmentum Obscurus may bear little resemblance to a Lunar-class Cruiser constructed above Kar Durniash in the Ultima Segmentum and even less so to a Lunar constructed among the vast shipyards of Mars' Ring of Iron. Nonetheless, they will all have roughly the same operating characteristics and weapons configuration, and thus can be easily serviced by any orbital facility throughout Imperial space.
Note there are also classes of Battlecruisers. Although based on a hull-design that is similar to the regular Cruiser types, these starships are generally somewhat larger and more heavily armed, incorporating more advanced power distribution systems capable of supporting Battleship-grade weaponry in a Cruiser hull. A notable example is the Mars-class Battlecruiser, with its fighter bays, broadside batteries, dorsal Lance turrets and an immense, prow-mounted Nova Cannon capable of attacking targets at extremely long range.
A subset of the Cruiser type is the Light Cruiser. These warships fall in size between Cruisers and Escorts, mixing the firepower and durability of the former with the speed and manoeuvrability of the latter. The Dauntless-class is a very common Light Cruiser class in the Imperial Navy. Extremely self-sufficient, it is fast and has enough firepower to be a threat to Escorts and larger capital ships alike. There are other classes of Light Cruiser in common use in the Imperial Navy, most notably the Endeavour-class, which serves in varied forms and hull types throughout the Imperium. Light Cruisers are a fairly common warship class, for they are relatively simple to construct for a capital ship, and they are uniquely suited for reconnaissance patrols and for ensuring a presence where their speed and manoeuvrability are an advantage, and where having a larger number of smaller hulls allows the Imperial Navy to monitor a wider area of space.
Ironclad
Ironclads, much like their contemporary Imperial Navy counterparts, the Battleships, are vast, 8-kilometre-long vessels which lack the Void Shielding of their Battleship counterparts in favour of metres of adamantium plate armour. These warships, built before the advent of Void Shield technology, have since been phased out of production by the Imperium to be replaced by more modern designs. However, those remaining in service have been recommissioned for a variety of purposes; various pattern Ironclads may be retrofitted with a gargantuan, ship-, station- and even planet-killer cannon running the entire length of the ship's keel, linked directly to the stern fusion reactors; others may simply be braced and reinforced for the purpose of ramming into -- and through -- enemy vessels. These starships are rare in the Imperial Navy, due to their archaic design and the lack of facilities still capable of repairing them at existing Forge Worlds, let alone manufacturing new ones.
Escort Ship
Escort ships are the smallest type of warship in the Imperial Navy's fleets, found in two distinct sub-classes. The larger of the two sub-classes of Escorts are Frigates, which are better armed and more heavily armoured than other Escorts. Destroyers are generally smaller than Frigates, but they are by far the fastest and most manoeuvrable interstellar warships employed by the Imperial Navy. They are usually organised in squadrons of 2 to 6 vessels and will always operate as a group or "wolf pack." The main task of Escorts is to serve as a screen for capital starships against enemy torpedoes and Attack Craft so that they can get into position more quickly and safely. They are also employed behind the gun line to finish off enemy Cruisers that have been damaged so that the larger vessels can concentrate on the most dangerous capital ship threats in an enemy formation. Most Escort classes specialise in a certain role, such as the Cobra-class torpedo boat Destroyers or the Firestorm-class Frigate with its armour-piercing prow-mounted Lance. Escorts are normally not more than 1.5 kilometres in length, with Destroyers generally being only 750 metres to 1.5 kilometres from prow to stern.