From Seo Wiki - Search Engine Optimization and Programming Languages
Pack200 is Sun's compression utility included in Java 1.5.0. It was designed for compressing jars and works most efficiently on Java class files. Using Pack200 compression can reduce the size of a jar by about 60%. By packing the jars placed on an update site and enabling update to unpack using unpack200 tool for those jars after download, the amount of data downloaded during an update can be greatly reduced.