How is ACS different from PTS and when does it replace PTS for FAA certification?
The Airman Certification Standards (ACS) is basically an 'enhanced' version of the Practical Test Standards (PTS). According to FAA briefing on ACS,


  It (ACS) adds task-specific knowledge and risk management elements to each
  PTS Area of Operation and Task.


In general, while the PTS concentrates on what the applicant must 'do', the ACS specifies what he/she must 'know' and 'consider' (in addition to the 'do' part) to qualify for a given airman certificate or rating. The differences (in draft) are shown in this slide.



Image from Update on the ACS by FAA

According to the FAA briefing,


  • The “skills” section of the ACS covers the flight proficiency performance
  metrics in today’s PTS.
  
  • For each PTS Area of Operation/Task, the ACS lists the elements of
  aeronautical knowledge that support that skill.
  
  • In addition, for each PTS Area of Operation/Task, the ACS lists the risk
  management elements or behaviors associated with it.


The ACS is still under development, but FAA is targeting a June 2016 implementation (for airplane) depending on results of prototype (there is one running at the Embry-Riddle Aeronautical University) and some other factors.