Vaastu is all about the interaction of various forms of energy pertaining to a structure. The forms of energy, as we know, classified as heat, light, magnetic, electric, sound, supersonic sound, ultraviolet radiations etc. all affects us directly or indirectly whether we subscribe to the concept or not.
Any form of energy under a set of circumstances produces a sense of comfort or uneasiness. Sometimes, we feel a particular house to have cheerful and pleasant atmosphere whereas, many often we might have felt some houses to have the atmosphere so gloomy and dull that you will count your time to get out of it at the first opportunity.
Plainly speaking the various forms of energy within these structures are imbalanced. Maintaining Balance is the primary condition of nature for survival.
Again if we will come to scientific part of it, Every object in the universe whether inanimate or animate including human beings are the manifestation of this great energy field. Hence every object in universe are related to each other in some way or other. thats why we are being affected by numerous objects in universe depending upon their energy radiation. Even planets, millions miles away, affect us in various ways(courtsey: Science of Astrology & Astronomy).
Coming to micro level, every object within a house affect each other, be it a fire place, water sink, overhead tank, toilet, living room, puja room etc. and again coming to animate aspects places for sleeping, sitting, dining, reading etc. even pets affects each other in various ways.
Hence it is imperative to arrange objects in such a manner so as to create a positive field of energy which is harmonious to our own energy level. Here Science of Vaastu comes to our help.
To maintain balance and stability, Nature always creates two opposite things simultaneously. While every problem has its remedy in nature, we only have to ascertain the correct procedure to our benefit.
e.g. a small change in your bed room arrangements, changes the overall energy field surrounding it, affecting us either positively or negatively. That will makes the difference in every aspects of your life.
With the impending restriction to the word limit, we cant do justice to the great science of Vaastu, which has been practiced from the time immemorial. Short-cut description may be misleading.