The Assault Cruiser is an Admiral (Tier 5) level Federation Cruiser. Like all Cruisers, bar the Light Cruiser, they also enjoy a bonus to their subsystem power granting +5 to Weapons, Engines, Shields and Auxiliary. The most well known Assault Cruiser is the U.S.S. Enterprise-E, which was commanded by former Captain made Ambassador Jean-Luc Picard. As of 2408 she was commanded by Captain Data.
(Link: http://sto.gamepedia.com/Assault_Cruiser w/photo)