These are the persons that do all of the ground handling (amazing deffinition). That is, they do the refueling, loading, de-icing and marshalling (guiding of the aircraft into and out of the parking area). This is usually done by employees of the airline, however in recent years, in attempt to save money, some airlines have contracted these services out to independent companies. Ironically, some of these "independent companies" are actually subsidiaries of other airlines... a distinct change from the way things were done historically...