Zyklon-B was a hydrocynic Acid, that when exposed to oxygen vaporizes and gives off a bitter odor. The substance appears in pellets and left blue stains on the chambers walls and can even be seen today.This was originally Intended for commercial use as disinfectant and an insecticide. The prisoners were forced into air tight chambers discised as shower rooms. The Guards would put on gas masks and administer the substance through special air shafts in the ceiling.
The final scene in boy in the striped pajamas shows an accurate representation of the Zyklon-b usage and there tactic for getting people into the chambers.
Zyklon-B was typically kept in stock piles like this, in special rooms.