The ARIA Engine is a 64-bit audio sampling and synthesis engine. It is developed by Plogue Art et Technologie Inc in collaboration with Garritan Corp. (now part of MakeMusic, Inc.). The file format is based on the SFZ 1.0 / SFZ 2.0 open file formats for instrument programming and the Scala open file format to define scales and temperaments. The ARIA Engine can be found in various software from Garritan, Plogue, AKAI, Sony, Microsoft and MakeMusic.
The ARIA Player is Garritan's user interface on top of the ARIA Engine. Much in the same way that an MP3 player (such as iTunes) allows you to play back MP3 files, the ARIA Player lets you load and play instruments from your Garritan library. And since all Garritan libraries use the ARIA Player, you can mix and match samples from any library within a single instance of the ARIA Player.
For Garritan users, updates to the ARIA Player and ARIA Engine are packaged together and can be downloaded here. There is no need to update the ARIA Engine separately, except when instructed to do so by support staff.