What is the meaning of "Om/Aum"?
When do people say it?
Where has it come from and what are it's effects?
Some say it is a very holy sound for Hindus because it represents the sound of the gods. They also believe it is origin of all sounds. Some believe "Om" is the Indian form of the "Word" in the Bible. It refers to the place of universal energy from which the universe has come from. Different people believe in different things.
Om/Aum begins and ends all prayers, mantras(chants) and bhajans(songs for god), and all of these are used to worship in a temple. Even in the beginnings and ends of the Vedas and Upanishads, Om/Aum is there. When someone recites OM, it is recognized as the universe and ultimate source of it, which is Brahman.
Om is a Sanskrit word, which originated at the same time as the Vedas. But the symbol is in a different language. That language is called Devanagari. If the word is repeated on and on, it creates a sound that emits in the form of a gentle, good and soothing resonance. The symbol produces a steady and calm influence on the mind, but only when visualized properly.