Designers tune the thermal characteristics of buildings so that they moderate external environmental conditions and maintain internal conditions using the minimum resources of materials and fuel.
Passive design maximises the use of 'natural' sources of heating, cooling and ventilation to create comfortable conditions inside buildings. It harness environmental conditions such as solar radiation, cool night air and air pressure differences to drive the internal environment. Passive measures do not involve mechanical or electrical systems.
Read the full article on the Designing Buildings Wiki.