A body bag is essentially a non-porous bag designed to contain a human corpse, while providing protection to those handling it from blood-borne pathogens and so forth. In films and TV, they are usually shown as black. However, most modern body bags are white - this makes it easier to see something that's dropped out of the cadaver's pockets or similar. The government keeps large reserves of body bags at strategic points for ease of cleanup after large natural disasters and manmade bloodbaths.
The expensive body bags (supplied to the US military and major hospitals and police departments) are made of a proprietary formula poly. Cheaper versions are often made from vinyl. High quality modern body bags contain no chlorides and carbons - these pollute the atmosphere when burned and may damage retort chambers in crematoriums. They are also designed to take low temperatures - below -20C - in case of use in adverse weather conditions, and for use in morgue coolers.