Cue the Imperial March! It's a Star Destroyer, officially an Imperial II-class or Imperator-class Star Destroyer.
Lego Star Destroyer:
Episodes IV-VI Star Destroyer:
I have a fantastic book called Star Wars: Complete Cross-Sections, and it says this about Imperial II-class Star Destroyers:
Class II Star Destroyers made up the fleet which captured the Rebel base on Hoth. Lord Vader led this fleet from the Executor, first of the Super Star Destroyers, over eight times as large as the Imperial-class ships.
So, you may have seen your Lego Star Destroyer in The Empire Strikes Back.
To give you an idea of the size of this ship, here is a diagram from evilzwaardfishy*:
The Imperial Star Destroyer seems pretty big, until you look at the bottom half of the diagram and see it compared to the Super Star Destroyer. You can also see the Millenium Falcon in the upper-right-hand corner of the diagram.
If you would like to know what the difference is between the Imperial I- and Imperial II-class Star Destroyers, the Hangarbay Technical Database offers this explanation:
The Imperator-class Star Destroyer (Mark II) is a newer version of the original ISD [Imperial Star Destroyer]. It has a faster hyperdrive, upgraded shields, and improved weapons. An example of the Mark II ISD would be the Avenger, as opposed to the original ISD, an example of which is the Devastator. They can easily be distinguished via the appearance of the engines: original ISDs have small triple baffles surrounding the main engines, whereas Mark II ISDs have ring baffles.
Clear as mud! Anyway, today's build is a Star Destroyer. :o)
*evilzwaardfishy site contains adult content
More information about the Imperial II-class Star Destroyer may be found at Wookieepedia.
Lego parts information for today's build may be found at Bricklink.