Technically, they mean the same thing - to lose consciousness. However, there is are situations where one is used more than the other. Usually, if a person passes out from being sick or from lack of air, we use the term 'faint' - especially when a woman is involved. It gives a picture of a little more dignity. If a person loses consciousness because they drank too much alcohol, we would be much more likely to say that they pass out. We would never say 'fainted' in such a situation, even though that is technically what happened. "Look at him. Five whiskeys and he's pass out on the floor." You could use 'passed out' in a situation where a person got too little air and you would be correct. Sometimes, the terms are interchangeable.

Five whiskeys and he's PASSED out on the floor "Faint" can also be used to describe a feeling. "She felt FAINT, so she went outside for some fresh air". Another usage of "faint": this describes something that is faded or lighter than normal. "The writing on the old letter was now so FAINT that it could not be easily read."