JAR stands for Java Archive, primarily associated with 'Java' by Sun Microsystems, Inc. These are compressed files based on the ZIP file format used by the Java Runtime Environment (JRE), a structure used for executing Java programs and it can accumulate many files into one. The files with the .jar extension hold Java classes and metadata. JAR files are often used by programs like Thunderbird and Firefox to store program add-ons and program themes. JAR files can also be created to store files in the same way as the ZIP files are used. The JAR files contain several files that have been compressed into a single folder or one large file that has been compacted to achieve a practicable file size. The JAR format also supports compression, which reduces the size of the file and improves download time still further. JAR is the only cross-platform archive format and a format that can handle audio & image files and class files too. JAR is written in java and is open standard and fully extendable.