First thing: forget about the FANS thing: it is not Future anymore. It has been for 15 years or more, but I hope the furure is here, now...
Second thing: check the ICAO doc PBN (performance based navigation) Manual, and also take a look at the ECACNAV page, which is more ilustrative. A lot of achronyms and concepts of concepts, all very conceptual.
But simply put:
In order to achieve the N part of CNS (comm, nav, surveillance) necessary fo the ATM (air traffic management), they have come up with PBN.
PBN specifications: RNAV or RNP, such as BRNAV, RNAV 5, RNP 1...
RNAV APCH: they are GNSS based, and can be LNAV only (with MDA, similar to GPS NPA approach) or LNAV/baroVNAV (with a DA, also called APV, approach with vertical guidance) or LNAV/VNAV (with DA, also known as LPV "localizer performance approach with vertical guidance" which will be very similar to flying an ILS).
The APV capability has been available in airliners for 20 years, but they still talk about future navigation systems???
The LPVs will need some form of DGPS augmentation, such as SBAS or GBAS. With GBAS we will enjoy CAT II and III.