"Star Destroyers" is a general term for the ships made by the Kuat Drive Yards corporation for the Galactic Empire in the Star Wars universe. They range from the Victory Star Destroyer to the Executor-class commandships.
There are many kinds of Imperial Star Destroyers (that is, "Star Destroyers" flying the Imperial flag. The Rebellion not only captured several Star Destroyers, as in the Dark Empire comic books, but also constructed its own type well after the movies take place). There are only a few seen onscreen, but many more have been invented either in the comic books (such as the Eclipse-class command ships) or in games. The most common ones are:
The Victory Star Destroyer. This did not appear in the movies, but was taken from a concept sketch. The backstory is that it was introduced during the Clone Wars. It is fairly small and slow, but there are many of them still around.
The Imperator Star Destroyer. This is what most people think of when they see "Imperial Star Destroyer". There are two types; the I (example the Devastator in A New Hope) and the II (example the Avenger in The Empire Strikes Back), with the II having more armor and weaponry. These were pretty much the backbone of the Imperial Navy, there being approximately 25,000 at the Empire's peak (ref. Specter of the Past).
The Executor. There were about 12 or so of these constructed over the span of the movies and novels, at various times and by various people (supposedly, there were 4 at the time of The Empire Strikes Back, with other people such as Warlord Zsinj or Superior General Delvardus). They are the longest ships in the Navy at 17,600 meters long (11 times the length of a regular ISD. Don't argue with me about this), and they are pretty much only around to look intimidating.
There are other weird ones, but those are the only types in the movie. More can be found at Curtis Saxtons' Star Wars Technical Commentaries (http://www.theforce.net/swtc/dagger.html).