A "virtual airline" is a computer simulation of an airline. People can join and fly routes on their PC's. All you need is some sort of flight simulation software, such as ATP (Airline Transport Pilot), Airline Simulator or Airline Simulator 2, or Flight Simulator. The airline has hubs, route structures, and an aircraft fleet. Pilots fly their routes each week and report back their results for a score (things like flight time and amount of fuel burned).