what's meant by .jar extension

Answer 1

the java archiving tool

Answer 2

.jar extention is a file used to easily transfer a java program from a pc to other. it is like a zip file which holds different files in it for easy transfer. it can be executable if it has a class which is executable i.e it can run without terminal window like in bluej.

Answer 3

jar also could be your custom library that you could use it in your programs.

Suppose you have coded a class that save and read data to/from a file. Then instead of rewriting the same class in every program that should read/write to a file..you could just import your library and start using it.