The Aggressor was an Imperial I-class Star Destroyer, specifically of the Imperial I class.
During the initial stages of the Galactic Civil War, the Aggressor provided transport for Mar Barezz, an agent of the Imperial Security Bureau. After Gil Bastra's death in 6.5 ABY, Kirtan Loor was transported to Coruscant by it.
In the novel X-Wing: Rogue Squadron, Loor's presence is depicted on a shuttle connected to the Super Star Destroyer (SSD) Aggressor. This is a mistake, as this particular Aggressor had been destroyed prior to that event. The Complete Star Wars Encyclopedia published in 2008 clarifies that Loor traveled on the Imperial Star Destroyer (ISD) Aggressor, and makes no mention of a connection to any Super Star Destroyer.