(jar) A compressed archive file containing java class files, filename extension: ".jar". The java development kit contains a tool called "jar" for creating .jar files, similar to the standard unix tar command. As well as archiving and compressing the Java class files, it also inserts a "manifest" file which can contain information about the class files, such as a digital signature. Combining class files into a single archive file makes it possible to download them in a single http transaction. This, and the compression, speeds up execution of Java programs delivered via the internet.