Milk in bags may also be aseptic (I think). This is pasteurized to higher temperatures and possibly longer so that the milk is fully shelf stable. It can sit on the shelf for years and not go bad. This makes it good for third-world countries where refrigeration may be a little harder to come by.
This milk tastes a little funny, but as with anything, it is an acquired taste. Yoo hoo may be a relation of sorts.