JAR is a file format for storing class files used for running Java programs. JAR stands for Java ARchive. Jar files are created and can be extracted by the JAR program. The Jar Program uses options similar to those used in Tar.
Since Java programs are often made up of many Class files programmers try to organise them in Jar Files. Where a group of classes may be used as a package (or library) The classes can be archived in the same Jar. Then the archive just has to be on the ClassPath to be used in another program.