Guest: a guest is whoever opens the application without having been authenticated.
Passenger: a passenger is an authenticated user who is capable of reserve a ride from a certain location to another. He’s also able to share a ride with other passengers or to look for shared rides.
Taxi driver: a taxi driver is an authenticated user who has been granted permission to confirm or not a reservation. In case of confirmation he should lock his availability from incoming reservations aside shared rides.
Administrator: an administrator is an authenticated user who has been granted supervising permissions. He’s also able to view, modify reservations’ states and access statistics and aggregated information about the application.