Typically it refers to the coding and programming side of web site production as opposed to the web design side.
JavaScript® (often shortened to JS) is a lightweight, interpreted, object-oriented language with first-class functions.
jQuery is a lightweight, "write less, do more", JavaScript library. The purpose of jQuery is to make it much easier to use JavaScript on your website.
PHP is an acronym for "PHP: Hypertext Preprocessor" · PHP is a widely-used, open source scripting language.