I was wondering if it is possible for other servers to send POST to my server?

For example lets say I have a page that requires certain POST parameters be set so that they can see the page. Is it possible for someone to write a simple script on their own site that sends these POST parameters to my site? Such as a transaction page that uses a $_POST['item'] and displays details on it.

If so, isn't this a huge security breach? How can I prevent this?