The mantra 'Om' or 'Aum' is the most sacred Hindu mantra. It is the Pranava Mantra, which gave birth to the whole Universe. It leads the way to the Brahman, the omnipresent, omnipotent, omniscient and the Formless Ultimate.
The word 'Aum' is profound in itself. It contains within itself the universal forces of creation, protection and destruction. It is the sound of Space and also that created by the seven spiritual centers (chakras) in the subtle body, which can be heard only by the Yogis and ascetics.
The 'Aum' also symbolizes the three states or gunas, namely, rajas, satva and tamas. It also includes the Triloka or the three worlds - heaven, earth and the Netherworld in itself.
The Gayathri Mantra, "Om Bhur Bhuvah SwAha" is just an extension of the divine 'Om'. Both these mantras are believed to lead man towards enlightenment and realizing the true nature of the Supreme Being, the ParamAtma, within himself.
The poster featured here depicts the brilliance of the 'Om', relating it with the purity and power of the Gayathri Mantra.