The new kit due to be in place at Shanwick later this year, (Gander sometime after that), will eventually be hooked up to an ADSB feed and will provide a pseudo radar picture enabling the controllers to 'see' your actual position on the basis of data received directly from the aircraft, as opposed to an approximate position calculated and displayed by the flight data processing ATC computer.
ADS at the moment provides controllers with waypoint position reporting as significant route points are crossed during the oceanic en-route phase. (Or when a change is made by the crew to the FMS). Position information can also be sent by the FMC, depending upon the set up of the aircraft itself. CPDLC position reports, as with ADS and FMC, only contain the LAT/LONG and ATA/ETA for the position. Some ATC centres are not set up to use CPDLC position reports and only use CPDLC to request changes in FL or Mach.
ADS and FMC position reports will tell controllers when you reach a particular route point stored in the FMS. ADS will also generate a report at an intermediate point should you alter the route held in the FMS. CPDLC will send a position report for your actual position at the exact time you send it. ADSB will send a series of reports at specific time intervals giving a constant update of your position every 30 seconds/2 minutes/5 minutes or whatever the downlink period is set to.
ADSB will enable Oceanic and other areas, for example parts of Australia and the Arctic region amongst others to employ separations similar to those of radar sectors without the need for ground based radar systems.
However, the accuaracy of the ATC systems such as the new one for Shanwick are such that controllers can pretty well 'see you' as things stand, without ADSB.
Hope this helps.