A Java(TM) application consists of one or more class files, one of which defines the main() method. You can run an application in any environment that provides a Java interpreter, for example, anywhere there's a Java IDE. The Java Runtime Environment (JRE) from Sun provides an interpreter as well, but omits the development-related tools such as the compiler.

A Java applet is not an application; it does not define main(). Instead, applets depend on host applications for start-up, windowing, and shut-down operations, typically, a Web browser:

Many applets simply render a graphical image in a designated area of the Web browser window; others provide a GUI with command buttons that initiate application-specific operations. Applets operate under several security restrictions, which protect users from unknowingly downloading applets that snoop for private data, damage local file systems, and so on.

Applet programming involves many Java concepts that are, by definition, beyond the scope of an introductory tutorial. A later topic in this tutorial includes an introduction to applets (Applets).