Functionality
Unlike Java applets, Web Start applications do not run inside the browser. However, by default they run in the same sandbox as applets, with several minor extensions like allowing to load and save the file that is explicitly selected by the user through the file selection dialog. Only signed applications can be configured to have additional or even all permissions.
Web Start has an advantage over applets in that it overcomes many compatibility problems with browsers' Java plugins and different JVM versions. On the other hand, Web Start programs are no longer part of the web page. They are independent applications that run in a separate frame.
Web Start can also launch unmodified applets that are packaged inside .jar files, by writing the appropriate JNLP file. This file can also pass the applet parameters. Such applets also run in a separate frame. Applet launcher may not support some specific cases like loading class as resource.
Like applets, Java Web Start is cross platform.
Read more about this topic: Java Web Start