A Star Destroyer is a class of large battleships used by the Galactic Empire in the Star Wars saga. Normally at about 1,600 meters in length, they are one of the largest Destroyer-class warships in the Empire's navy. Bristling with turbolasers, laser cannons, and also complimenting dozens of fighters and bombers, it has a multirole purpose as a flagship, warship, carrier and transport, also carrying about a battalion of troops for ground engagements.
Those are the Imperial and Imperial II-class, however. Several predecessors are the Venator-class and the Victory-class. The Venator came first, at 1,137 meters, and holding several dozen starfighters. The Venator was designed before the Empire came to power, during the Galactic Republic. It has 8 powerful heavy turbolasers, but other than that and several other laser cannons, it has weak armor compared to later warships. The Victory is the smallest Star Destroyer in operational history, at 900 meters. Carrying a rather low compliment of single-ships, it is designed mainly for combat against heavier ships and cruisers.
After the Galactic Empire was dissolved into the Imperial Remnant, the New Republic, it's biggest enemy, developed it's own version of Star Destroyer- the Republic-class. At a slightly smaller size than the Imperial at 1,250 meters, it is more efficient, effective and economical as it's predecessors. It carries the generally same amount of starfighters and has even more weaponry.