The signature restaurants are the ones Disney specifically puts forwards as their fine dining options. They have world-class food and service, and apply a dress code. (The vast majority of restaurants at WDW are casual dining.)
The signature restaurants are:
Artist Point
California Grill
Citricos
Flying Fish
Hollywood Brown Derby (no dress code)
Narcoossee
In addition, Bistro de Paris is considered to be in this same class. Victoria and Alberts is considered to be in an ever higher class.