Mode S (S = Select) provides for downlinked parameters (DAP's) as used in TC currently. Also, its the eventual replacement for Mode A SSR code use. The 4 digit octal SSR code is replaced by a unique 24 bit address. The pilot enters his RT callsign into the FMS - hopefully correctly! DAP's include heading, track, ground speed, 25' altitude updates, selected altitude etc.
Mode S will provide a considerable improvement in redusing RF clutter and garbling. ADS-B (Airborne Dependent Surveillance - Broadcast) relies on GPS data - ok in the wide open spaces but not really accurate enough for European skies - yet. Maybe some applications out there, the middle of oz for instance.