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.
Armament layout on these ships follows from their physical shape. The main armament is placed on either side of the dorsal tiers. 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 smaller weapons for defence against smaller craft.
Imperial-class Star Destroyer
The standard mile-long ISD, the Imperial-class Star Destroyer is 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 <something> of TIE fighters. 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
Um... I dunno.