Passive building design is a way of designing a building so that the natural elements work in accordance with how that building is used.
For example a home built with passive design design philosophies in mind would be constructed so that the climate of the area helps to maintain a comfortable climate for the occupants of a home. This can be done by orienting a home so that the sun shines into the living spaces, amplifying the heating of the home, if it is needed.
An important consideration for passively designed buildings is that the passive design elements can be adapted and/or controlled in some way. While a passive building makes the most out of natural elements that are always working, sometimes these elements are not in tandem with what works well at certain times. In these instances there needs to be some kind of counter-measure to the original design to offset the issue.