It depends on how the form is coded. In other words, you can have just plain form-controls such as text input boxes, textareas, etc. In that case, once the form is 'submitted', the information entered will disappear.
But often forms are coded so that the form-controls 'retain state', meaning that they retain the information entered even after the form submit button is hit. This is done as a convenience for the user so that if there is a validation error and the user is sent back to the form to fix it, he doesn't have to complete the whole form all over again. He just has to make the correction.
Usually a user is redirected away from the form to a 'success' message on a different page if the form is submitted successfully. If they then go back to the form, it will no longer contain the information entered.