A Java Applet is an application designed to run within a web page. Applets can operate across different computer operating systems because they rely upon a Java Runtime Environment (JRE) implemented by a specific computer’s web-browsing application.
Java Applets are secure and safe to use. One should always be wary of downloading applications from anonymous sources on the Internet because of the possibility of viruses. However, the Java language explicitly puts limits on the functionality of applets so as to prohibit malicious behavior. A Java applet cannot read or write from a user’s computer without express permission from the user.