It's not the Stargazer or the Prometheus types, but the kind that looks like someone took an Intrepid class ship and attached an extra pair of nacelles on it. It's in a few of the loading screens and at the Battlegroup Omega place in space.
I had always played an engineer and thought it was some sort of Science ship, but I was just looking through the ships on the web page and didn't see anything like it at all. Is it something that is coming out soon? Or is it around here somewhere and I'm not seeing it?
Otherwise, without a picture or a better description, I don't know what you're looking for.
Edit: given the last poster's comments, you may be looking for the Typhoon-class, which as he/she/it said, is an NPC ship. The Terran Empire fly a few of 'em, and Undine try to steal one in one of the PvE missions.