The Om, or Aum, is a holy symbol in Buddhism and Hinduism. It marks the beginning and end of a prayer or ritual, and is even thought to be the sound of creation itself.
It also represents a kind of Hindu trinity called the Trimurti. The Trimurti represents creation from start to finish, with Brahma as the creator, Vishnu as the sustainer, and Shiva as the destroyer. In this manner the om represents all of creation.