From memory so please excuse duff gen!
ADS = Automatic dependant surveillance
In short it is a continuous automatic data link between ATC and the aircraft giving information like altitude, speed, position etc of aircraft.
If I'm not mistaken Johannesburg uses the Eurocat 1000 system. It may be upgraded to 2000 which is the same as Mauritius, Australia and Antananarivo (?) use for control of their FIR's. These systems are ADSB compatible.
After having spoken to the ATC in the new area control centre in Mauritius they say that if aircraft are ADSB equipped they can reduce seperation significantly below that which is required during procedural control. Of course it becomes almost useless if you are being handed over to an ATS unit that is not ADSB equipped - where normal seperation would be applicable again. A single aircraft using ADSB could likely be relieved from making some position reports.
Information can be found in the text section of the Jepp. If no one else answers you in detail by the time I return from Paris on Saturday, I'll jot down the info and post it on Saturday.
Hope my memory has not left me open to too much criticism!
edited for spelling