PHP is an HTML-embedded scripting language. Much of its syntax is borrowed from C, Java and Perl with a couple of unique PHP-specific features thrown in. The goal of the language is to allow web developers to write dynamically generated pages quickly.
PHP stands for PHP: Hypertext Preprocessor. This confuses many people because the first word of the acronym is the acronym. This type of acronym is called a recursive acronym. For more information, the curious can visit Free On-Line Dictionary of Computing or the Wikipedia entry on recursive acronyms.