When you pass out, you become unconscious. You could be hit, or maybe you was ill, badly hurt or drunk.
As for faint, you are becoming unconscious, that is, you are so weak that is about to become unconscious.
For example, you may be feeling faint with hunger.
Other sense of it is, when someone (an example) receives bad news and suddenly become unconscious.
Anyway, faint/fainting suggest that before passing out, the person gets weak, unsteady, and pass out for some seconds.
If I understood it well, anyway more comments are welcome. I am always open to learn more.