Java Archive, jar, file. Jar files are zip files on steroids. In addition to an applet's class files, a Java archive contains a manifest listing the archive's contents and encrypted security information such as the applet vendor.
In the event of a "Class not found" error message, it is useful to open a jar file with a standard file decompression utility, such as WinZip, and see if the requested file is actually available.
Class files are, of course, also distributed by Netscape, Sun and Microsoft as part of a browser package and it is possible that a "Class not found" error is due to the fact that a browser vendor does not support the necessary functionality.