(i have already posted this under meditation & mind control section, just reposting so that maximum people can benefit)

Mantra involves repeating a pure sound vibration. The goal of this repetition is inner peace – harmony of mind, body, and soul. A strong Vedic principle about chanting mantras is "the more done, the more benefits we will receive". If one has extreme problems or they want to profoundly alter their karma, mantras can be performed for longer periods of time on a daily basis. Only thing is that a person should be aware what the mantra he uses is intented for & how to properly chant that else meaning & vibration of mantra changes
gayatri mantra is one of the most reverred in HINDU & BUDDHIST culture

AUM BHOOR BHUWAH SWAHA,
TAT SAVITUR VARENYAM
BHARGO DEVASAYA DHEEMAHI
DHIYO YO NAHA PRACHODAYAT

single word by word meaning
AUM:primordial sound,the Godhead, Supreme Deity|Bhoor: earth or physical plane|Bhuvah: water or mental/astral plane. Swaha: fire or celestial plane |tat - that (referring to God)|savitur:the all-pervading Consciousness|vareNyaM - most excellent, fit to be worshipped| bhargo -  splendor (the light that bestows understanding)|devasya - divine, of the deity|dhîmahi -  we meditate upon| dhiyo - prayer, intuition, understanding of Reality |yo - he who, the one who|nah - our, of us|
prachodayât - may he energize, direct, inspire, guide, unfold... or he who energizes, directs, inspires, guides, unfolds

Complete meaning
We meditate on the glory of the Creator;
Who has created the Universe;
Who is worthy of Worship;
Who is the embodiment of Knowledge and Light;
Who is the remover of all Sin and Ignorance;
May He enlighten our Intellect.

this mantra is one of the best for gaining knowledge, because we all have come to the world for learning & finally GOD is knowledge and love (only thru true knowledge can anyone show unconditional love towards anything)