A Java applet is an applet written in the Java programming language. They can run in a web browser, or in Sun's AppletViewer, a stand alone tool to test applets.
Applets are used to provide interactive features to web applications that can not be provided by HTML. They are executed in a sandbox by most web browsers, preventing them to access local data. The code of the applet is downloaded from a web server and the browser either embeds the applet into a web page or opens a new window showing the applet's user interface. Since Java's bytecode is platform independent, Java applets can be viewed by browsers for many platforms, including Windows, Unix, MacOS or Linux.
For applets to be compliant with MS Windows operating systems, applet code has to be developed using Java compatible with Microsoft JVM or else the object tag can be used to ensure a more modern JVM has been downloaded and installed from Sun.
See also: Java Servlet