Taken directly from PHP's home, PHP.net, "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."
This is generally a good definition of PHP. However, it does contain a lot of terms you may not be used to. Another way to think of PHP is a powerful, behind the scenes scripting language that your visitors won't see!
When someone visits your PHP webpage, your web server processes the PHP code. It then sees which parts it needs to show to visitors(content and pictures) and hides the other stuff(file operations, math calculations, etc.) then translates your PHP into HTML. After the translation into HTML, it sends the webpage to your visitor's web browser.
It is also helpful to think of PHP in terms of what it can do for you. PHP will allow you to:
Reduce the time to create large websites.
Create a customized user experience for visitors based on information that you have gathered from them.
Open up thousands of possibilities for online tools. Check out PHP - HotScripts for examples of the great things that are possible with PHP.
Allow creation of shopping carts for e-commerce websites.