Type rating

A type rating is a licensing agency's certification of an airplane pilot to fly a certain aircraft type that requires additional training beyond the scope of the initial license and aircraft class training.
International Regulation.
The International Civil Aviation Organization (ICAO) specifies the international personnel licensing requirements, as documented in Annex 1 to the Convention on International Civil Aviation. Which aircraft require a type rating is decided by the National Aviation Authority, in accordance with specifications outlined by ICAO.
ICAO stipulates that:
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 the European 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 requires 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.
Many commercial aircraft share type ratings, allowing qualified pilots to operate both of them with minimal transition training. Examples include the Boeing 757 and Boeing 767, Boeing 777 and Boeing 787, Airbus A330 and Airbus A340, Airbus A330 and Airbus A350 and all of the members of the A320 family (the A318, A319, A320, and A321).
Type Ratings in the United States.
In order to remain complaint with ICAO Annex 1, the Federal Aviation Administration (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 outlined in Code of Federal Regulations Title 14 §61.55 (14 CFR 61.55) and introduced in 2006. Such a type rating is "not" required for operations completely within the United States.
An instrument rating is required for some type ratings.
In the United States some type ratings can be issued with a "visual flight rules (VFR) only" limitation when the type rating checkride was conducted without instrument flight rules (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.)
Type Ratings for EASA states.
European Aviation Safety Agency (EASA) publishes its ICAO type rating compliance and further regulations in Part-FCL - Flight Crew Licensing, Subpart H.