BOD is Biochemical Oxygen ...

BOD is Biochemical Oxygen Demand which signifies the amount of Oxygen needed by the micro organisms like bacteria and virus to digest the organic matter in the waste water (or used water) measured after 5 days when the used water sample is incubated at 20 degree celsius. It was infact developed as a measure of microbial contamination of the River Thames in London about a century ago when the river was heavily contaminated with faecal matter and the citizens were bearing a severe disease burden due to this contamination. It took about 5 days for the contaminated water to travel its entire distance to the sea and thus the 5 day BOD.

BOD is widely used for assessing the usabliity of used water for different purposes. It also indicates indirectly the amount of dissolved oxygen present in the used water  for the aquatic organisms to sustain and indicates the quality of surface waters, although DO directly indicates the quality of surface waters. Hope this gives an idea my friend.