Users can use client-side extensions for many purposes, but one of their main functions is to perform input field validations. A helper application is a stand-alone program that runs on the user's PC and is invoked by the Web browser. Pluggable applications are like helper applications in that their purpose is to process and display data that the browser cannot handle directly. Java Applets are compiled programs that are downloaded when an HTML page is requested and are then run by the browser. Applets run as bytecode interpreted programs, which reduces the likelihood that they will transmit a virus, since each instruction is validated before being run.
Scripts are programs embedded in an HTML page.
Scripts integrate well with the Web browser because they add functionality without changing the look and feel of a standard Web page.