To put it simply, PHP (Pre-Hypertext Processing) allows you to write code that generates HTML pages on the fly. It's a very powerful language that is able to interface with many other net apps, especially databases such as Postgres and MySQL (just to name a couple). The user never sees the PHP code pages, only the output. Here's a "Hello World" example: <?php echo "Hello World!"; ?> So if you have a file called 'index.php' on the web server with that code in it, you will have the result of a page displayed to the user with just Hello World on it whenever someone accesses that page. Personally, I love PHP, it's free, powerful, and has a huge community behind it. Many websites, forums, wikis, etc., use PHP.

1) "PHP (PHP: Hypertext Preprocessor) is a programming language that is put into HTML to make it work. It lets you make web pages that can change. For example, they can say different things depending on how a person uses the web page." Source: http://simple.wikipedia.org/wiki/PHP 2) "PHP is a popular programming language for extending web pages with dynamic features. While plain-vanilla HTML can lay out an attractive page and perhaps present forms for users to enter information, HTML can't actually do anything with the data that the user enters in the form. This is where web server extension languages like PHP come in, providing a way to handle form submissions and other user requests by accessing databases, sending email, generating images on the fly and performing other actions. PHP is currently the most popular web server extension language, used by many websites both large and small. Its popularity is partly due to its free, open-source nature and partly due to its friendliness and convenience. Tasks such as reading an entire file and outputting it to the web browser can be accomplished with a single line of PHP code. And PHP programmers can begin by sprinkling a small amount of code into a page otherwise made up entirely of HTML— a convenience also available in Microsoft's ASP.NET and other extension languages." Source and further information: http://www.boutell.com/newfaq/definitions/php.html