What data does ACARS send back to base? Can it be used to track a plane?
Can ACARS be turned off? Would this generate a warning at the base station?
  


Yes, ACARS can be turned off.  See another answer of mine for more details.  Typically other systems use ACARS to send reports on a regular basis to update position and track other information, plus on an "as-needed" basis for abnormal things.  It would be up to the individual airline to look for these reports and to generate a warning if one didn't check in at the expected time, but I don't believe that they do this.  There is a lot of variation in the communication systems and they aren't 100% reliable so a lot of false alarms would be triggered.


  
  Can ACARS send postion, altitude and heading information automatically?
  


ACARS itself can not, but other systems like the Flight Management Systems (FMS) or ADS-C on board the aircraft can use ACARS to send reports like this automatically, and in some cases they do.


  
  Can ACARS be pinged to track an aircraft's position and heading? Would this require any intervention by the pilots?
  


Again, ACARS is simply a communication system.  A message can be sent via ACARS which will forward to the appropriate system requesting this information and have it respond.


  
  Is this system standard on commercial airliners?
  


I believe that most airliners that travel over international waters have ACARS, but I'm not sure if it's a requirement or not.


  
  What data do Airlines collect from the ACARS, if any at all?
  


This varies from airline to airline, but messages at the beginning and end of the flight (used to track flight time, etc.) and periodically during the flight are typical.  Usually periodic position updates and abnormal system indications are included as well.