Transfer service is an alternative to booking a taxi to/from an airport in a foreign country. It is possible to order a transfer to travel between cities or tourist locations, but most often this is a service to get from/to a transport hub: an airport, a train or bus station, a sea or river port.
The booking should be made in advance: different car classes in different countries have their own minimum time for booking, between 16 and 24 hours. Preliminary booking gives the carrier time to get prepared for the trip, which explains the advantages of private transfers.