Pranava or the first vibration ever, is the word Om / Aum. It is also referred to as the Omkara. Indian philosophy considers it the basis all creation. Pranava represents the Supreme Brahman or creator, and the Self within everyone. Pranava is also a mantra, which can be recited, for meditation and self realization. Elaboration on the meaning, and usage of Pranava, can be found in scriptures like the Upanishads and Shiva Puran.
Om / Aum is pronounced subtly with a single syllable Om, or with a long tone Aum, with three syllables A, U, and M. The short subtle Pranava is believed to be for those who live a mundane life, while the long one is for those who are more spiritually inclined.
A gross Pranava with 5 syllables 'Om Namah Shivaya' is believed to be more for the active person.