Platform Independence on WWW
A so called Plug-In (an application program) can be downloaded to the client (WWW-browser). The Plug-In is run under the client Operating-System (OS) as any application program.
An Java Applet is downloaded from the sever and run in a Java Virtual Machine, JVM. The applet is OS platform independent and can do no harm to the client as it is run isolated from the OS.
If the 'applet' is run on the on the server it is called a servlet (reduced access time due to applet down load to client not needed).