The Department of Transportation breaks airlines down into three categories based on annual revenue. An international airline is an airline that generates more than a billion dollars per year. There are 14 airlines classified as international in the U.S. as of November 2010. Domestic airlines have revenues between $100 million and one billion dollars, but some fly between countries. The third classification is regional airlines, with revenues under $100 million and additional specifications like the size of fleet and types of planes.
A second type of classification is by level of service. The Skytrax Airline Star-Ranking System evaluates all airlines and awards a rating between one and five stars. The rating is derived from examining 800 areas of service and products offered. In 2010, there were only six five-star-rated airlines worldwide.
Airlines have their own internal classifications for seating and rewards programs, with particular flight routes offering economy, business and first-class services and with different rewards and mileage programs.
