A virtual airline is a virtual company that works nearly in the same way as a real airline. Pilots in a virtual airline fly using flight simulators for computers, so basically anyone with a computer can be a virtual pilot.
Virtual Airlines requires much work from their owners: painting aircrafts in the company livery, building up schedules, maintaine the web site, manage the pilots and much more. VA Services are web sites which supports the virtual airlines with their operations in many ways.