In literal terms, vastu means 'to dwell' or 'to exist' and shastra means science. There are two words associated with this science - vastu and vaastu. Vastu means pure, subtle energy and vaastu means embodied material energy.
When vastu or subtle energy goes into vibration of its own, many objects of nature such as buildings, temples, idols, etc. come into being. These material forms are called vaastu. Today, vaastu is commonly used to denote buildings.

The science of vastu is based on the concept of space (subtle energy) as a dynamic element out of which all objects of nature come into existence and into which all disappear ultimately. The frequency of vibration of this subtle energy, which is based on a particular rhythm or time measure, causes the birth and development of all objects of nature. So time is the causal element of all material forms.

The main aim of vastu science is to create building spaces to live in harmony with subtle nature.
The inner space of an individual and the outer space in the cosmos are vibrating at a particular frequency or rhythm. If a part of the vast space is isolated and enclosed by a four-walled  structure, called a building, it becomes a living organism and the enclosed space starts vibrating at a particular frequency. The architect trained in the science and technology of vastu and vaastu designs the building in such a way that its vibrations are numerically equal to the vibrations of the occupants of the building which is determined by their birth stars.

This is made possible by a special calculation maintained in vaastu technology. The building encloses a part of the cosmic space which is filled with particles of  energy each in the form of 'Vastu Purusha Mandala' where'Vastu Purusha' means material energy and 'Mandala' means diagram. This energy grid pattern consists of 9 x 9 = 81 squares and is the basic  layout for all building spaces.

Vaastu viyda has gone abroad and as you know, its Chinese counterpart, Feng-Shui, is very popular in the West. On my recent visit to Sri Lanka, I noticed that their houses and monasteries were designed on the basis of the Vaastu Purusha Mandala. The city of Guatemala has also applied the concept of the Vaastu Purusha Mandala as the basic plan for their city layout, design of buildings and pyramids.
When I visited Mexico, I noticed that the Mayan pyramids resembled the gopurams (pyramids) of South Indian temples.

The design and layout of the temple, idols and selection of site of Indian temples is based on vastu.
Temples are always constructed on a square or rectangular plot of land, this helps to confer spiritual peace to the temple goers. This rule is also applicable to houses and other buildings. The central core, Brahmasthan (belonging to Brahma, the Creator) is left open. The subtle God is always in the middle of the Brahmasthan. When God is manifested in idol form, it must be placed a little away from the center in a direction moving away from the main entrance. Ferocious deities such as Kali are located at the periphery of the temple as they are considered too gross. The only God that occupies the center of the temple is Shiva Linga, not even Lord Shiva.

Mathematically speaking, the temple and the idol in the temple are of the same mould. A temple is built in the shape of a human God by slightly modifying the structure using certain vibrational measures. Hence it is said that the temple is not merely the home of God but the form of God as well.