The methods of the forms tell you how the variables are processed. In the address here for the forums you will see something like "thread=12345". In the GET method any variable located after the question mark in the address bar is accessible. In the POST method (which is a little more secure) the variables are still available to a server-side script, but because the data does not come from the address bar it requires that the form be sent through the server preventing people from guessing at what variables would be. For example a bad GET form would have password=hackme in the address bar. See what I mean?
There's not much to it really.
<form action="URL-to-the-form-processing-script-on-your-web-server.php" method="post">
Form action property: