external style sheets are .css files that are linked to your HTML file using @import or <link /> tags, as opposed to inline style sheets which are written inside of your HTML using a...
You would use an external stylesheet anytime the styles you've written are going to be used on more than one page. For instance, let's pretend that you're managing a site with several hundred pages....
to implement javascript embed the statements between <script> n </script>,,, and include any scripting language.eg:<script LANGUAGE="javascript"> block of codes </script>