In GTA San Andreas, the Emperor is based on an early-90's Infiniti Q45. In GTA IV, it's based off of the '77-'79 Cadillac Deville or Fleetwood (with a flattened front end and no hood ornament).
GTA San Andreas
It is commonly found in San Fierro, and is used in Wear Flowers in Your Hair, the first mission in San Fierro where CJ hires Dwaine and Jethro to work in the garage in Doherty. It is also found in Grand Theft Auto IV.
Acceleration and speed are somewhat disappointing, as are handling and braking. The engine sounds as though it is a large V6 or V8, but is apparently incapable of delivering substantial power to the bulky, rear-drive vehicle. Using the E-brake around corners almost guarantees fishtailing, making this car almost useless when driving at high speeds. It is, however, a sturdy vehicle, and does a fairly good job of absorbing damage.
GTA IV Rendition
In GTA IV, it is based on a '77-'79 Cadillac Deville or Fleetwood Cadillac Deville or Fleetwood. It features a "beater" version of the Emperor, with junky paint finishes, broken seats and old wheels and tires.