PHP, which requires a file end with a .php or similar extension, is entirely server-side, meant for having the server provide you with a static page. A good example of this is my main page, http://floatzel.net/index2.php. The content there is updated server-side, which pulls information from this forum. The PHP code is meant to provide the user with an HTML (or XHTML) document.
JavaScript, which can either be embedded in another web document or in its own .js file, is the most popular programming language of the Web. This allows you to interact with a page, so when you click a button that says "Select All", all the checkboxes on a list get ticked. A good example of this can be found here, http://floatzel.net/pokemon/whosthatpokemon.php. (Don't let the .php extension fool you; I used that to grab a list of the Pokémon from a database until I realized an external JavaScript file put far less strain on the server.) That page uses JavaScript to control a simple quiz game with images and a timer.
