Java Web Start is framework developed by Sun Microsystems that enables starting Java applications directly from the web using a browser. Unlike Java applets, Webstart applications do not run inside the browser, and the sandbox in which they run does not have to be as restricted, although this can be configured. One chief advantage of web start over applets is overcoming many compatibility problems with browsers' Java plug-ins and different JVM versions. On the other hand, Web Start programs cannot communicate with the browser as easily as applets.
Version 1.0 of Web Start was introduced by Sun in March 2001, and since J2SE version 1.4 it has been included by default with the JRE and does not have to be installed separately.
JNLP (Java Network Launching Protocol) is a closely-related concept that is often used interchangeably with the term "Web Start". It is the protocol, defined as an XML file format, that specifies how Java Web Start applications are started. JNLP files include information such as where the jar package is located and what is the main class of the application, as well as any parameters for the program. With the right browser configuration, JNLP files are passed to a Java runtime environment which downloads the application onto the user's machine and starts executing it. As well as standalone applications, JNLP can be used to start Java applets.
Important Web Start features include caching the downloaded application files locally, the ability to automatically download and install JREs in case the user does not have Java installed (although this does not work with every browser and platform), and for programmers to specify what JRE version is needed to run the program.