Java - Applet
An applet is a small client application written in the Java programming language that executes in the Java virtual machine installed in the Web browser.
However, client systems will likely need the Java Plug-in and possibly a security policy file in order for the applet to successfully execute in the Web browser.
When you use a Java technology-enabled browser to view a page that contains an applet, the applet's code is transferred to your system and executed by the browser's Java Virtual Machine (JVM).
Applets are created from classes but do not have a main method to start them. Instead, they have several methods to control their execution.
Applet vs Web Component
Web components are the preferred API for creating a Web client program because no plug-ins or security policy files are needed on the client systems.
Also, Web components enable cleaner and more modular application design because they provide a way to separate applications programming from Web page design. Personnel involved in Web page design thus do not need to understand Java programming language syntax to do their jobs.