An applet is small program that is written in Java and it's designed to be placed on a web page. Applets cannot be consider full applications because they usually have one or two features and are created to be very accessible.
To create an applet, you need a development environment (a program to write code, a compiler to transform and so on). To learn more about this issue, I suggest you check the following website where you will find examples with explanations, tips and more.
