What is the difference between an aviation training device (ATD) and a flight training device (FTD)? According to FAR 61.57, the regulation requires different experience requirements when a pilot uses FTD or ATD for their experience minimums. So I have tried to look up the difference between those two but I don't really understand.

According to Advisory Circular 61-136A, the definition of ATD is a training device, other than a FFS or FTD, that includes


  a replica of aircraft instruments, equipment, panels, and controls in an open flight deck area or an enclosed aircraft cockpit.


The definition of FTD, according to FAR Part 1, is also 


  a replica of aircraft instruments, equipment, panels, and controls in an open flight deck area or an enclosed aircraft cockpit


They look pretty much same to me.

Could you give me the examples of those two?
14 CFR Part 141, Chapter 41 Flight simulators, flight training devices, and training aids. Requirements for a FTD:


  (1) Be a full-size replica of instruments, equipment panels, and
  controls of an aircraft, or set of aircraft, in an open flight deck
  area or in an enclosed cockpit, including the hardware and software
  for the systems installed that is necessary to simulate the aircraft
  in ground and flight operations;


The important mention is it must be a full-size replica of controls and panels. This doesn't apply for an ATD (see definition in AC 61-136A, appendix 2).

See also:


Sorting the simulators.
FAA withdraws ATD rule.