PHP
If you've been to a website that prompts you to login, you've probably encountered a server-side scripting language. Due to its market saturation, this means you've probably come across PHP. PHP was designed by Rasmus Lerdorf to display his resume online and to collect data from his visitors.
Basically, PHP allows a static webpage to become dynamic. "PHP" is an acronym that stands for "PHP: Hypertext Preprocessor". The word "Preprocessor" means that PHP makes changes before the HTML page is created. This enables developers to create powerful applications that can publish a blog, remotely control hardware, or run a powerful website such as Wikipedia or Wikibooks. Of course, to accomplish something such as this, you need a database application such as MySQL.