Applet

An applet is a mini-program that will run only under a Web browser. The applet is downloaded automatically as part of a Web page. When the applet is activated it executes a program. This is part of its beauty – it provides you with a way to automatically distribute the client software from the server at the time the user needs the client software, and no sooner. They get the latest version of the client software without fail and without difficult re-installation. Because of the way
Java is designed, the programmer needs to create only a single program, and that program automatically works with all computers that have browsers with built-in Java interpreters.

Applets are Java programs that execute within the context of a Web page. They interact with the user while his Web page is active and stop their execution when his Web page is no longer active. Applets are valuable because they are simple to use. A user only needs to open an applet's Web page to download and execute an applet.

Java applets, are run from inside a browser. A reference to an applet is embedded in a Web page using a special HTML tag. When a reader, using a Java-enabled browser, loads a Web page with an applet in it, the browser downloads that applet from a Web server and executes it on the local system Because Java applets run inside a Java browser, they have access to the structure the browser provides: an existing window, an event-handling and graphics context, and the surrounding user interface.

One advantage a Java applet has over a scripted program is that it’s in compiled form, so the source code isn’t available to the client.

Applet security is a major concern among Web users and applet developers. From a user's perspective, an exploitable applet security flaw could result in sensitive data being modified or disclosed, or their computer being rendered inoperable. From a developer's perspective, strong applet security is necessary to make Web users comfortable with using applets. However, too high a level of security limits their applets' capabilities.