Technical Information
Java applets are executed in a sandbox by most web browsers, preventing them from accessing local data like clipboard or file system. 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.
A Java applet extends the class java.applet.Applet, or in the case of a Swing applet, javax.swing.JApplet. The class must override methods from the applet class to set up a user interface inside itself (Applet is a descendant of Panel which is a descendant of Container. As applet inherits from container, it has largely the same user interface possibilities as an ordinary Java application, including regions with user specific visualization.
The first implementations involved downloading an applet class by class. While classes are small files, there are frequently a lot of them, so applets got a reputation as slow loading components. However, since jars were introduced, an applet is usually delivered as a single file that has a size similar to a large image file (hundreds of kilobytes to several megabytes).
The domain from where the applet executable has been downloaded is the only domain to which the usual (unsigned) applet is allowed to communicate. This domain can be different from the domain where the surrounding HTML document is hosted.
Java system libraries and runtimes are backwards compatible, allowing to write code that runs both on current and on future versions of the Java virtual machine.
Read more about this topic: Java Applet
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