I really don't know the difference as I don't even know what PHP does.
A web page (i.e. html, xhtml) is just static, but a browser only knows how to display html. So for content that is dynamic, such as forums, you need some method of converting that dynamic content into the static output required by a browser. PHP is one method of converting dynamic content into a static page that can be displayed by the browser. The different versions specified are different setups for being able to interpret php on the server to generate the webpage that will be displayed to the end user. In most cases you'll want the FastCGI as this will improve the performance of your site unless there's something in the default php.ini your host is giving you that you need to override that you can't do with .htaccess.