Passive solar refers to means of using sunlight for energy without active mechanical systems (as contrasted to active solar). Such technologies convert sunlight into usable heat (water, air, thermal mass), cause air-movement for ventilating, or store heat for future use, without the assistance of other energy sources.
A solarium on the equator-side of a building is one common example. Passive solar also is used to describe technology and design principles to reduce summer cooling requirements.  Passive cooling is a subset of passive solar technology.
Technologies that use a significant amount of conventional energy to power pumps or fans are classified as active solar technologies. Some passive systems use a small amount of conventional energy to control dampers, shutters, night insulation, and other devices that enhance solar energy collection, storage, use, and reduce undesirable heat transfer.
Passive solar technologies include direct and indirect solar gain for space heating, primarily using south facing windows and skylights. Other forms of passive solar are solar water heating systems based on the thermosiphon, use of thermal mass and phase-change materials for slowing indoor air temperature swings, solar cookers, the solar chimney for enhancing natural ventilation, and earth sheltering which are effective ways of gaining the natural solar energy for use in your home or business.
More widely, passive solar includes technologies such as the solar furnace and solar forge, but these typically require some external energy for aligning their concentrating mirrors or receivers, and historically have not proven to be practical or cost effective for wide-spread use. 'Low-grade' energy needs, such as space and water heating, have proven, over time, to be better applications for passive solar energy utilization. Passive solar design should be a part of construction of every new home or addition.