PBW stands for Powder Brewery Wash, which is a non-hazardous buffered alkaline brewery cleaner. It is a cleaner of choice for many breweries where it outperforms more hazardous caustic chemicals without removing the flesh from your bones. It will not corrode soft metals. Even better deal on bulk quantity. Comes in a jar with a screw top lid.
When mixing hot water and PBW in an air or water-tight space, it will create pressure. Please use caution when using PBW in enclosed equipment.