JAR format name stands for Java ARchive and it usually comprises of multiple Java files.
This format is used for designing and distributing Mozilla add-ons for enhancing compatible software.
In order to be used, these add-ons need to dragged and dropped onto the Add-ons window of the dedicated host application (Firefox, Thunderbird, SeaMonkey or Sunbird).
JAR format is also used by software developers for distributing their applications. These apps can only be launched and run on platforms that have Java Runtime Environment installed.