Feedback messages help the user stay informed about the status of the task. As too many messages can also be disruptive, they should be used sparingly and be relevant to the completion of the task.
Form elements which have an error are marked with a border and an error message underneath or next to the input field. The page should scroll to the first input field with an error on the page.
Inline messages display a validation/warning/error that concerns the whole task flow, rather than a single text input field. They usually appear at the top of the page or of the content area that is concerned.