Pack200, specified in JSR 200, is a compacting archive format developed by Sun, reducing JAR file sizes by a factor of 7 to 9. Pack200 may also refer to the Pack200 compression tools (pack200 and unpack200) provided in Sun's Java Development Kit since 1.5.0, as well as the Pack200 compressed files.
After Pack200, usually a GZIP compression is applied. In HTTP compression, such content is specified by the Content-Encoding type "pack200-gzip". When stored as a file, the extension ".pack.gz" is used.
Pack200 is optimized for compressing JAR archive files, specifically the Java bytecode portion of the JAR file. Application of this technology includes faster Java application deployment over Java Web Start.
References
Pack200 Wikipedia(Text) CC BY-SA