The term "Hypertext Preprocessor" says that PHP is meant to be used to process web pages or HTML (Hypertext Markup Language) documents. It can be used for many different purposes such as processing web forms, sending email, handling files, and interacting with a database such as MySQL. Since it is so widely used, it is part of two more acronyms--LAMP (Linux, Apache, MySQL and PHP) and WAMP (Windows, Apache, MySQL and PHP). The LAMP stack is the most used open source web development stack today. All parts of it are licensed under various open source licenses and are free for anyone to download and use.
PHP is a very robust language, but it is still relatively easy to learn. Beginners can create dynamic web pages with a few lines of code. On the other hand, it is hearty enough to create websites such as Facebook and Yahoo! and is the language used to create both Wordpress and Joomla.