Imperial Star Destroyer
Imperial Star Destroyers (or ISDs) have many design elements in common, the most obvious of which is the streamlined 'arrow' shape. This allows an aggressive, forward-facing attitude to bring many weapons to bear on a target. The upper and lower armour belts end at the side 'slots', lightly armoured areas where the internal honeycomb of the ship is exposed. The basic physical layout of an arrow-head with dorsal tiers of 'crellenations' or superstructure surmounted by a distinctive KDY bridge tower is common across a great many types, classes and sizes of Imperial warships.
Many of these vessels also have one or more hangars on the ventral surface, for the handling of starfighters and various planetary landing craft. Others have clean ventral lines, trading the utility of smaller craft for superior armour coverage. Many classes have powerplants too large to fit entirely within the physical bounds of the armour belts, and there is thus a larger or smaller 'bulb' on the ventral surface, to protect the oversized reactor.
Armament layout on these ships follows from their physical shape. The main armament of turbolasers is placed on either side of the dorsal tiers, in large turrets containing between two and eight individual weapons. The secondary armament is distributed between the side 'slots' (which often feature 'notches' in the upper and lower armour belts to improve fire arcs) and batteries placed along the centerline of the ship. The entire surface of the ship is covered in lighter weapons for defence against smaller craft. Imperial starships have a great deal of redundancy built into their armament - the emplacements are controlled locally, and the larger weapons often have dedicated reactors and fire control systems for use when the main systems are unavailable. This allows the warship's armament to be effective, even after significant damage.
The ships are protected by a powerful shielding system and heavy armour on many surfaces. The shield system protects the ship from both physical impacts and energy weapons (through ray shields), and this protection is limited in the first case by the internal mounting of the shield system and in the second by the capacity of the heat-sinks which absorb the incoming energy. This protection is effective for some time in regular combat - an ISD can survive half an hour of combat with a similar foe.
Imperial-class Star Destroyer
The standard mile-long (1.6km) ISD is the Imperial-class Star Destroyer, a versatile small warship. It was originally called the Imperator-class Star Destroyer before being renamed after the Jedi purge. It combines powerful sublight engines with a flexible turbolaser armament and a wing of 72 TIE craft. There are sub-variants of this class, known as Imperial I-class (ISD-I, sometimes referred to as the Devastator-class) and the Imperial II-class (ISD-II, or the Avenger-class).
Tector-class Star Destroyer
The Tector-class Star Destroyer is a hangarless Star Destoyer. These ships opted for more armor plating, their reactor bulb reinforced and not visible. Because the Tector-class was more armored it was much more effective in combat than their Imperial-class cousins, however, this also left the Tector-class without any form of fighter support.
Dominator-class Star Destroyer
Allegiance-class Star Destroyer
Allegiance-class Star Destroyers are a significant departure from the basic wedge-destroyer design. The hull is longer (at ~2.2km), but narrower, and the superstructure is more heavily armoured and sloped. It possesses no ventral hangars, and a significantly larger ventral bulb, suggesting a much more powerful powerplant.
The Allegiance is thus a more specialised design, like Tector, in that it is focused entirely on ship-to-ship combat. It appears in battles as a central vessel or flagship, with numerous smaller ships (including Imperial-class Star Destroyers) for support.