A type rating is an allowance to fly a certain aircraft type that requires additional training beyond the scope of initial license and aircraft class training. What aircraft require a type rating is decided by the local aviation authority. In many countries pilots of single-engined aircraft under a certain maximum weight (5,700 kg or 12,500 lb, typically) do not require a type rating for each model, all or most such aircraft being covered by one class rating instead. There are exceptions to this, e.g. under Joint Aviation Authorities (JAA) regulations the piston version of the Piper Malibu does require its own type rating.
In New Zealand and South Africa there is no class rating, each aircraft model requiring its own rating. Countries which have adopted the class rating system for small aircraft typically require additional training and license endorsement for complexity features such as conventional undercarriage (tailwheels), variable pitch propellers, retractable undercarriage, etc.
Starting in 2006, the FAA in the United States requires co-pilots (second-in-command, or SIC) to have a 'SIC Type Rating' for aircraft requiring a crew of two, and otherwise requires a type rating to act as pilot-in-command (PIC) to fly internationally, or over international airspace. This is in order to remain compliant with ICAO. This is outlined in 14 CFR 61.55. Such a type rating is NOT required for operations completely within the United States. Obtaining a SIC Type Ratings is significantly less rigorous than obtaining a 'full' or PIC type rating.
An instrument rating is required for some type ratings.
In the U.S. some type ratings can be issued with a "VFR Only" limitation when the type rating checkride was conducted without IFR approaches or operations, but only VFR maneuvers and procedures. This is most typical in older aircraft (i.e. Ford Trimotor, N-B25, B17, etc