From Lunarpages Web Hosting Wiki
Java applets are client-side scripts that are created locally on a system using Java language and then compiled and uploaded. A users browser determines support for these or not. The program to allow applets to run on a browser is included on most computer systems, although people can disable the capability or not update their program and so not be able to run newer applets. Again, because this is client side, all webhosts can support this.
JSP and Servlets
JSP and Servelets are server-side scripting that requires the user's webhost to provide a servlet runner and support for the languages. Our servlet runner is Resin, and it at an additional cost. You would need to send a request to firstname.lastname@example.org and provide your Lunarpages username, last 4 digits of your credit card on file, domain or subdomain to enable JSP onto, and authorization to do so.
For JSP and servlets, a users browser does not need anything turned on or enabled to run pages coded using them because the server is what parses them, so pages built using this would run on any browser so long as you have JSP and servlet support on the account. JSP and servlets are written using Java language, so knowledge of Java is required to successfully use these.