JAR: Jar stands for Java Archive.
A jar file usually has file name extension .jar. It contains mainly java class files but any types of files can be included, e.g., XML files, HTML files, properties files, gif/jpg image files, text files, PDF files, and binary files, and so on.
What is the difference between a jar file and a zip file?
Basically, a jar file is the same as a zip file, except that the jar file contains a META-INF directory to store metadata or attributes.
The most well-known file is META-INF/MANIFEST.MF. You can customize the content of a MANIFEST.MF when creating a jar file, and if you don't, a default one is generated and included.
Created-By: 1.5.0_06 (Sun Microsystems Inc.)
It's perfectly fine to have other configuration files and directories under META-INF.
Advantages of JARs:
This greatly improves the speed with which an applet can be loaded onto a web page and begin functioning. The JAR format also supports compression, which reduces the size of the file and improves download time still further.