There are many different styles of breathwork, but they all fall into two general categories: Meditative or Integrative.
The key difference is the time spend continuously breathing. Integrative breathwork is anything less than 8 minutes. Meditative is breathing for 8 or more minutes without any holds or breaks. When breathing continuously for longer periods of time, the mind is able to unhook and relax allowing your body and mind to relax into a meditative state.