Passive solar building design makes the sun an integral part of lighting, heating and cooling the building. The "passive" part of this type of design means that it doesn't require any sort of device, and unlike something like solar panels, it doesn't take a big initial investment or a long time to pay off. It's complicated, though; designers must carefully choose everything from the location, size, and type of windows and walls to the plants placed around the building for shade.
It starts with looking at the sun's path as its rays strike the building. Sure, you know that the sun rises in the East and sets in the West. It also "moves" across the sky during the day, and it's in a different position in the sky depending on the time of year. There are also a ton of other factors that go into the design, including insolation -- the amount of solar radiation absorbed by a surface area. That's not just influenced by the sun's rays, either; it also has to do with climate. Wind patterns, clouds, humidity -- they all make a difference in how hot or cold a building gets on the inside. Designers usually employ software to calculate the best placement and materials, and it's different for every building.
In general, a building using passive solar design has large windows and deciduous plants on the southern side (to let in maximum sunlight in the winter and shade in the summer). It also uses building materials that have high thermal mass -- meaning that they retain heat well -- like concrete and tile. Passive solar design cuts down on the energy used for the building's HVAC system as well as lighting. Having a commercial space lit by daylight is another mood-lifter, too.