Vessel Traffic Service :
VTS are shore-side systems which range from the provision of simple information messages to ships, such as position of other traffic or meterological hazard warnings, to extensive management of traffic within a port or waterway.
Generally, ships entering a VTS area report to the authorities, usually by radio, and may be tracked by the VTS control centre.
Ships must keep watch on a specific frequency for navigational or other warnings, while they may be contacted directly by the VTS operator if there is risk of an incident or, in areas where traffic flow is regulated, to be given advice on when to proceed.
More information here : http://www.imo.org/en/OurWork/Safety/Navigation/Pages/VesselTrafficServices.aspx