YANTRAS are geometric drawings that make visible the patterns of sound energy in a mantra. The Sanskrit word 'Yantra' derives from the root 'Yam' meaning to sustain or hold and streamlining or centralizing the energy inherent in a particular element or object. The suffix 'Tra' in Sanskrit means instrument. In order to facilitate concentration by focusing our entire energy on magic symbols the Yantras are made as vehicles of mantras for attaining power. In ancient texts Lord Shiva is supposed to have explained the mystical meaning of the Yantras to Goddess Parvati that, the Yantra is as essential to a god as oil is to light the lamp or as a body is to a living human being.
Yantra is a highly efficient tool for contemplation, concentration and meditation because it is a symbolic representation of aspects of divinity or cosmic forces. The Yantra provides a focal point that is a window into the absolute. When the mind is concentrated on a single, simple object (like 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. Other purpose is to enhance the quality of life, to attain success in business, to attract prosperity and abundance, even love, to heal and relieve health problems, to protect oneself from negative forces and so on.
Yantras can be drawn, engraved or painted on Gold, Silver, Copper, Crystal, Birch, paper and Vishnu Stone (Shaligrama) and sometimes on Earth (Rangoli) for decoration. Yantras can be worshipped or worn. If drawn on paper the colors preferable are red, orange, yellow or a combination of these. They can be drawn to whatever size is required. In worship they should be placed level on a pedestal or pitha. Before any Yantra is a suitable object for puja, it must be given life (pranapratishta), after that bathe it with milk, orange juice and honey on full moon days, light a candle and incense and pray to it. You should offer a flower and a raisin or other fruits. Each Yantra has its own visible power pattern and when combined with mantra, it builds form. The power of a Yantra, when made 'alive' through Mantra or other rituals is limitless. The mantra is the mind consciousness while the Yantra is the form of the deity. Yantras energized most effectively and powerfuly during the nights of 'Maha Shivratri', 'Holi' and 'Diwali' etc. Normally the related Mantras are also chanted in the presence of the Yantras for quick result. They must be cleaned and empowered periodically by the user.
Elements of Yantras
The most important aspects of Yantras are the elements (dot, triangles, circles,squares and star) as they represent cosmic energy. The seeker meditates on the central point or the dot, which is surrounded by the other elements such as triangles, circles, and squares.
The dot or the bindu has great significance. Generally placed at the center of the Yantra, the bindu is the focal point of concentration and must be as small as possible. It represents the one point from where the entire universe originated and represents the potential of all creation.
Upward-pointing equilateral triangles can variously represent the Passive Principle or Purusha Linga (the male phallic symbol), male energy or water.
Downward-pointing equilateral triangle, on the other hand, represents the Active Principle or Prakriti, Yoni (the female reproductive organ or womb), female energy or fire.
The Star formed by the combination of two triangles, is common in Yantras. It represents the union of Shiva and Shakti.
The Square is usually the outer limit of the Yantra, and represents the earth element.
Representing the air element, the circle also refers to the various solar plexi within the body and movement of energy.
Meaning of Numbers in Yantras
1: Absolute
6: Material access
2: Duality
7 :The man Dominates subject
3: 2+1= Perfection
0:Circle, God
There are four basic types of Yantras
Shakti Yantras
It is most prominent Yantras of deities (these are usually forms of the Great Mother or the Mahavidyas - 'Sources of supreme knowledge').
Example: Shree Yantra, Lakshmi Yantra, Kali Yantra, Durga Yantra etc.
Astrological Yantras
Used to harness the energies of the nine major planets. 
Example: Yantra for Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, Rahu, Ketu.
Architectural Yantras
Used for the ground plans of temples. 
Numerical Yantras
Mathematical Yantras are based on a particular sequence.