Romulan D'deridex - This is the ship commonly seen. It is a warbird, designed to look threatening but a lot of it is just space. [TNG D'deridex ]
Mogai Escort/Valdore Warship - Possibly the same ship class, possibly variants, possibly different. I only know of the Mogai Escort from Star Trek Online. The 2 ships that assisted the Enterprise-E at the battle with the IRW Scimitar were Valdore class. The labels fast, maneuverable, bigger version of the Bird of Prey with a strange configeration can probably be applied to both these ships.. [STO Mogai Escort ] [Nemesis Valdore ]
Romulan Scout - Appears in a TNG episode. Basic scout ship. [TNG Romulan Scout ]
Scimitar - This is a deadly warbird with Thalaron Radiation weapon included. It is a highly armed and armoured "predator". The first meeting with this ship showed it could fire while cloaked and at that time the Federation had no information on it and so could not combat it. The Enterprise-E confronted the IKS Scimitar in Star Trek: Nemesis.
(THIS LIST IS NOT COMPLETE)
For more and better info see: http://en.memory-alpha.org/wiki/Category:Romulan_starship_classes