Imperial Star Destroyer
Imperial Star Destroyers are the standard small starship type of the Imperial Starfleet, and an instantly recognisable symbol of the Emperors authority. Many variants and classes exist.
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.