Action attribute in forms is not a url. Why? When I check the action attribute of a drupal form, I never see a valid URL. What I see in the html code is a simple string, which usually matches the node title of the form. Given that the action attribute should contain a valid URL pointing to a server-side script, I am really confused! What are the mechanics behind the form's submission? Thanks in advance for your answers!
If you look again, I would hazard a guess that you are using pathauto, and that the default pattern for Webform paths is [node:title]. If you set it to form/[node:title] then the action attribute would look more like a path. eg. My Form would have an action attribute of action="/form/my-form".

In Drupal everything goes through the Drupal bootstrap. There are no calls to cgi-bin or the equivalent. Rendering a form or processing a form are done by calling the same url, which tells Drupal where to look for the form. What Drupal does depends upon the op associated with the call, and the rendering, validation, processing and storage are all defined by the modules underlying the form.