To breathe out...or in?

I have been to the gym and seen people breathing differently while lifting weights. So when do you breathe in and when do you breathe out?

Answer: The moment of time when you are exerting the most force, BREATHE OUT! EXHALE! Just like boxing..when we are punching out, we as according to Boss, we must "tsssss" like a snake to exert more force. Same goes to lifting. You realise that in a power lifting competition, the competitors keep making weird and loud noises. It somehow gives them more strength...so yea...breathe out when u are exerting force.