Yantra is a mystical instrument, which acts as a 
great cosmic conductor of energy, having symbolic 
diagrammatic 
representation of a Deity. It is used for harmony, prosperity, success, 
good health, wealth, yoga and meditation. Yantras are also referred as 
the abodes of the divine powers of God.
Yantra literally means "instrument". A Yantra is a geometric design 
acting as a highly efficient tool for contemplation, concentration and 
meditation. Yantras carry spiritual significance, and point the user to 
higher levels of consciousness.
The Yantra provides a focal point that is a window into the absolute.
 When the mind is concentrated on a single, simple object (in this case a
 Yantra), the mental chatter ceases. Eventually, the object is dropped 
when the mind can remain empty and silent without help. In the most 
advanced phases, 
it is possible to attain union with God by the geometric visualization of a Yantra.
The Yantra is like a microcosmic picture of the macrocosm. It 
is a focusing point and an outer and inner doorway. The Yantras are 
often focused on a specific deity and so by tuning into the different 
Yantras you can tap into certain deities or creative force centers in 
the universe. Yantras are as powerful as chanting the mantras. Yantras 
are continous recitation of mantras on your part.
Yantras are usually designed so that the eye is carried into the 
center, and very often they are symmetrical. They can be drawn on paper,
 wood, metal, or earth, or they can be three-dimensional. Yantras are 
very complex engines of energy containing geometric figures, 
permutations of mystical archetypal designs and embedded with energetic 
signatures. When activated or energized, the pre-programmed Yantra 
becomes "alive" so that the intentions or purpose contained within the 
Yantra are manifested in the physical.
Yantra may be used to represent the astronomical position of the 
planets over a given date and time. It is considered auspicious in Hindu
 mythology. These yantras are made up on various objects i.e. Paper, 
Precious stones, Metal Plates and alloys. It is believed that constantly
 concentrating on the representation helps to build fortunes, as planets
 have their peculiar gravity which governs basic emotions and karma. 
These yantras are often made on a particular date and time according to 
procedures defined in the vedas.
Concept of Yantra:
The Sanskrit word 'Yantra' derives from the root 'yam' meaning to 
sustain, hold or support the energy inherent in a particular element, 
object or concept. It is usually composed of an outer geometrical form 
enclosing interior geometrical designs. Each particular design is used 
for a specific purpose or intent. In its first meaning, 'Yantra' may 
refer to any kind of mechanical contrivance, which is harnessed to aid 
an enterprise. A Yantra in this sense, therefore, is any sort of machine
 or instrument such as is used in architecture, astronomy, alchemy, 
chemistry, warfare or recreation.
A Sanskrit text of the eleventh century AD, Samarangana Sutradhara on
 the science of architecture, gives vivid descriptions of the making and
 operating of such mechanical Yantra as a wooden flying bird, wooden 
aero planes meant to fly with hot mercury as fuel, male and female robot
 figures, etc. The meaning of the term Yantra has been expanded to refer
 to religious enterprises, and has acquired a special theological 
significance. Mystic Yantra are aids to and the chief instruments of 
meditative discipline. Basically a Yantra used in this context and for 
this purpose is an abstract geometrical design intended as a 'tool' for 
meditation and increased awareness.
Principles of Yantra:
Mystic Yantra are an amalgam of three principles:
The form principle (
Akriti-rupa).
The function-principle (
Kriya-rupa).
The power-principle (
Shakti-rupa).
They are, first of all, believed to reveal the inner basis of the 
forms and shapes abounding in the universe. Just as, whatever the outer 
structure, all matter is made of an intrinsic basic unity, the atom, so 
each aspect of the world can be seen in its structural form as a Yantra.
 As the scientist sees the final picture of the world in the orderly, 
simple, atomic structures in which certain primal shapes appear as a 
harmonized 'whole', so the Indian shilpi-Yogins (makers of ritual art) 
seek to identify the innermost structure of the universe by 
concentrating the variegated picture of world-appearances through 
intense yogic vision into simple form-equations. A Yantra, then, can be 
considered an ultimate form-equation of a specific energy manifesting in
 the world. These simple form-equations are held to epitomize the real 
nature of the cosmos as abstracted from the concrete.
In its widest application, 
Akriti-rupa refers to the
 inner or hidden form of structures, so that any structure, from an atom
 to a star, has its Akriti-rupa Yantra. Thus a flower or a leaf has an 
outer structure which is immediately perceptible, but it also has an 
inner form, which generally consists of a skeletal framework in which 
all its linear forms intersect with a central axis or nucleus: all forms
 have a gross structure and a 'subtle' inner structure, with a basic 
causal pattern (the inner form) for the external form.
Yantra function as revelatory symbols of cosmic truths
 and as instructional charts of the spiritual aspect of human 
experience. All the primal shapes of a Yantra are psychological symbols 
corresponding to inner states of human consciousness, through which 
control and expansion of psychic forces are possible. It is for this 
reason that a Yantra is said to embody a 'function-principle' (
Kriya-rupa).
By constant reinforcement in ritual worship the apparently inert 
Yantra-forms shake off their dormancy and act together as emblems of 
psychic power. In this case, the Yantra is said to move beyond 'form' 
and 'function' and emerges as a 'power diagram' (
Sakti-rupa)
 endowed with a self-generating propensity to transform a mundane 
experience into a psychic one. It is at this point that the Yantra is 
said to be 'revealed'. Although its outward meaning may be relatively 
easy to understand, the inner meaning that gives it its efficacy is 
difficult to grasp because its archetypal forms are basically concerned 
with the inner facts of psychic experience, gained through intuitive 
vision.
How do Yantras work ?
At the basis of Yantra operation is something called "shape energy" or 
"form energy". The idea is that every shape emits a very specific 
frequency and energy pattern. Examples of old believes in shape energy 
are the Yantras and mandalas of eastern philosophies, the star of David,
 the five pointed star (pentagon), the pyramids and so on. Certain 
'powers' are ascribed to the various shapes. Some have 'evil' or 
negative energies and some 'good' or positive energies, but in Yantra 
Yoga only the benefic and harmonious energies are used.

 When one focuses on a Yantra, his mind is atomatically "tuned in" by 
resonance into the specific form energy of that Yantra. The process of 
resonance is then maintained and amplified. The Yantra acts only as a 
"tune in" mechanism or a doorway. The subtle energy does not come from 
the Yantra itself, but from the macrocosm.
Basically 
Yantras are secret keys for establishing resonance
 with the benefic energies of the macrocosm. Very often the Yantras can 
put us in contact with extremely elevated energies and entities, being 
of invaluable help on the spiritual path.
A Sri Yantra is a type of mandala with geometric patterns. Sri Yantra
 literally means loom, instrument or machine. In actual practice, a 
yantra is a symbolic representation of aspects of divinity, the creative
 forces of the universe. It is an interlocking matrix of geometric 
figures, circles, triangles and floral patterns that form fractal 
patterns of elegance and beauty. These visual patterns can have a 
powerful effect on the mind. Just as primordial sounds, or mantras, can 
be useful in balancing our mind and body through hearing, primordial 
shapes can generate increased coherence in our brains, creating a 
balancing and calming influence. In cultures around the world, beautiful
 visual patterns are used to quiet a restless mind.
Importance of Yantras:
The Yantras are used for various purposes such as to diminish the ill
 effects of the nine planets, to attain good health and prosperity, to 
enhance the quality of one's life, attract prosperity, abundance, love, 
harmony, peace, enhance learning, promote healing, relieve health 
problems, for career advancement, achieve desired goals in projects, 
improve business, remove obstacles and protection from negative forces.
In ancient texts, 
Lord Shiva is supposed to have explained the mystical meaning of the Yantra to his consort, 
Goddess Parvati: "
The Yantra is as essential to a god as oil is to the oil lamp or as a body is to a living human being".
Yantras are also used for innumerable mundane purposes such as to 
enhance the quality of one's life, attract prosperity, abundance, love, 
harmony, peace, enhance learning, promote healing, relieve health 
problems, for career advancement, achieve desired goals in projects, 
improve business and protection from negative forces.
Text acquired from: http://www.vedicvaani.com/yantras