HTML stands for Hyper Text Markup Language. It is a language that web browsers are able to understand, and then translate into a page that is viewable in your browser. HTML first began back in 1991 with the introduction of a few tags. Since then it has grown into different versions and is now replaced in many areas by the very popular but very similar XHTML.
You will find an HTML file beginning with the <html> and closing with an </html> closing tag.
PHP stands for Hypertext Preprocessor (don’t worry, there won’t be a quiz on this!). PHP is a programming language for websites, that communicates with the server (Apache) and parses information returning it as HTML (or so you can display it as HTML).
PHP allows you to echo something that is dynamic or changing (like a date) unto a static html page (which doesn’t change). So rather than having to change the date on a static normal html page every single day, you could just make a PHP script to echo the date for you (and it would always be updated, granted the server is up to speed).
PHP is not seen by the user, but only by the admin of the site when viewing the code of the webpage. When viewed in a browser by a visitor to the site, the PHP code is first processed by the Apache server, and then outputted to the browser along with the HTML code, which can be mixed with PHP on a single page. Most PHP webpages have the .php extension, but servers can be set up to parse other extensions (.htm, .html, etc.) as PHP also (using Handlers).