Form usage and PHP scripting

I’m new to WordPress, and am finding it difficult to find resources covering how WordPress works under the hood. Specifically, in trying to write custom forms in PHP, it appears to me that WordPress is completely voiding out the $_POST array, forcing me to send forms via $_GET. If I use post to a WordPress managed page, and print_r($GLOBALS), I see that the POST array is completely empty.

Is there a way to change this behavior? If so, where can I find documentation about how to do it?

Thanks for you help,
Josh