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. Many of these vessels also have one or more hangars on the ventral surface, for the handling of starfighters and various planetary landing 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.