The Interdictor cruiser is a craft from the Star Wars
universe, and is a special variant of the Star Destroyer
design, specialised for a support and anti-insurgent role. It was designed by Seinar Fleet Systems
under the model name "Immobiliser 418" (1) Its main claim to fame is in 4 large gravity well generators
which appear as massive bulges in its otherwise sleek hull.
When powered up, these generators create the same hyperspace mass shadow as a planet, making it impossible for craft to enter hyperspace. The practical application of this is twofold, not only is this useful for preventing an enemy force from retreating, but they can also pull craft out of hyperspace, creating great opportunities for ambushes. Given the rebel habit of relying on hit and run tactics using hyperspace-capable fighters, an interdictor cruiser is an extreme threat, capable of trapping the rebel craft in-system and enable them to be hunted down. However, Interdictor cruisers are relatively rare, since they are very expensive, and have only a fraction of the size and firepower of other Star Destroyer varieties, leaving them to normally operate in support of other forces.
Its exact specifications are (2):
Length: 600m (compared to 1600m for an Imperial Star Destroyer)
Armaments: 20 laser cannons, 4 gravity well generators. (later variants added a tractor beam, 2 ion cannons, and 4 turbolasers).
Fighter Complement: 12 squadrons of TIE Fighters.
Note that the Interdictor cruiser never appears in any of the movies, so this craft is strictly "extended universe", but has appeared in countless Star Wars novels, as well as in a number of their space sim games.