Yantras are certain geometrical patterns inscribed with Mantras and certain alphabets and numbers on pure metal plates, made of Gold, Silver, or Copper. Earlier, they were made on Palm leaves. Along with the geometrical patterns, there are some alphabets inscribed which have certain astral energies, and also the Mantra related to the deity or planets. These Yantras are said to have a lot of power, to dispel evil. Actually, these Yantras are to be made on certain days, under certain constellations. They should be worshipped everyday, reciting the Mantra inscribed on them.
In the great Hindu epic called ‘Mahabharata', there is a story about a king named Karan. He was said to be a very large hearted soul. If anyone asked him for anything, he would never refuse. Once, during the battle of Mahabharata, the Pandavas (the good and righteous) realized that Karan, who was with the opponents side, was never going to be defeated and killed since he was wearing a special Kavacha (Yantra Talisman) which was so powerful that no one could kill him. So they managed to get the Yantra from Karan and only after that, could they defeat him. So, this only goes to demonstrate the power of the Yantra's shakti.
Some other Yantras that are available are, each for all nine planets, a single yantra for all nine planets called Navgraha yantra, yantras to overcome some great evil, yantra for Gods like Ganesh, Gayatri, Lakshmi, Shiva, Hanuman, Durga, Mahakali etc. Yantras are also made for achieving particular goals, like getting divine knowledge, mastery over some subject, for business, for VAASTU defects(Original ancient Hindu science, which deals with astral energies at homes, land and their favourable directions, like Feng Shui) for overcoming enemies, for diseases etc. Of course just keeping the yantra with you is not going to help. One must do the prescribed mantra and religious procedure to activate the yantra. Most of the mantras have to be repeatedly said day after day for a prescribed number or period. Only then it is said to work wonders.
The important thing is to do them religiously and have faith. You also need to follow other dos and don'ts as mentioned in the scriptures. Your state of mind, your heart, your deeds, your thoughts must all be good. If you are doing a Mantra for self healing, but have bad intentions for others, then it would negate all spiritual effects. Then you cannot complain that the Yantra and Mantra did not work. Rather than trying to wish bad for someone causing you distress, pray for the person to see righteousness, or just to help you get out the situation peacefully. Our mental involvement with material senses of love, hate, anger, attachment and ego are the cause of grief in our lives. Therefore when the mind is devoid of these poisons, and is as calm as a lake, then divine intervention takes place.
Spiritual healing comes in a complete package, or deal. You can't choose what you like and forego the rest. Your attitude, approach, intention and application must be all positively directed. Always keep your intelligence, practical commonsense and wisdom intact. Make your own effort first, then let God do the rest.