A gas chamber is an airtight room used for killing people or animals with poison gas.  The people or animals are put into the gas chamber, the door is sealed airtight from the outside, and a gas, such as carbon monoxide or hydrogen cyanide, is put into the chamber. The victims die from inhaling the poisonous gas.

In the United States 

In the United States, prisons have used gas chambers to execute prisoners who have been sentenced to death.  The first person in the United States to be executed in a gas chamber was Gee Jon, in a prison in Nevada, in 1924.  The government of Nevada did not want to use an electric chair, and thought hanging was cruel and inhumane.

The gas chamber is usually a small chamber, which is made air-tight with a door (typically with rubber strips around) into which the prisoner is escorted and either placed or fastened to a chair with straps, and left inside. After this, the gas is inserted either through a hose or a pellet is dropped into a solution which combined produces deadly gas, and the person(s) is asphyxiated within minutes. In the United States, one or two people maximum were executed, although recently it has become extremely uncommon. After California stopped using it in favour of lethal injection, it became nearly redundant. But it is still possible in several states, typically as a second method, in case there would not be drugs for an injection to be performed, or if the prisoner prefers to be executed by gas for any reason.

Recently, there has been proposals and a law has been enacted in Oklahoma for a variation, so-called nitrogen gas, either by chamber or a mask. It is believed to be less painful and anxious than the method previously used. In the beginning, actually, the idea of a gas chamber was to let the prisoner condemned to death sleep in a cell which would then be exposed to lethal gas during the night, without the prisoner's knowledge. However, because of poor isolation techniques it was too difficult and so the gas chamber was introduced instead. It was introduced in several states, including Mississippi, North Carolina, Missouri, Colorado, California, Arizona and New Mexico, but it never was as popular as the electric chair and later execution by injection. The method is in fact expensive and thought to be painful because the prisoner must breathe deeply to hasten the death, which is difficult when the body wants to reject the fumes. (With nitrogen, it is different because the body doesn't reject it as strange or fatal.) It is also extremely dangerous to the audience and personnel in case there is a leak, and impossible to halt in case there is an attempt to stop the execution, such as in the cases of Caryl Chessman and Burton Abbott. In the case of an electric current or injection the switch can be turned off and the needle ripped out, and while it might not be enough there is no risk for the people trying to save the prisoner. In one case, when the mechanism jammed, the executioner had to drop the pellet into the acid (producing the deadly fumes beneath the chair where the prisoner was sitting) and jump out of the chamber as the warden stood ready to slam the door and tighten it.

By the 1990s, the gas chamber had become unpopular in the United States.  It never became as popular as execution by electric chair or (more recently) lethal injection. When Donald Eugene Harding was executed in Arizona in 1992, witnesses described his death as "violent" and an "ugly event". The last prisoner to be executed in the gas chamber in the United States was Walter LaGrand, in Arizona in 1999. All states that still have the death penalty use lethal injection.

However, in April 2015, Oklahoma made a law using nitrogen gas to execute people, although it's not certain whether the gas would be used by a mask or similar contraption or in a gas chamber.  They approved this after it took over an hour to execute Clayton Lockett by lethal injection.

In Nazi Germany 

During The Holocaust, Nazi Germany used gas chambers to kill between two and three million people.

Nazi Germany first used gas chambers to kill people with disabilities.  The Nazis thought that people of disabilities were "life unworthy of life," and did not deserve to live.  Between 1939 and 1941, the Nazis killed 70,273 people with disabilities at hospitals like Hartheim Euthanasia Centre.

Nazi Germany planned to kill all of the Jewish people.  They called this plan the Final Solution.  In 1942, the Nazi regime held a meeting where it was decided to kill all Jewish people that fell under German control.  After this, they started constructing death camps with gas chambers.  In 1942, the Nazis started killing hundreds of people at a time in gas chambers at the Belzec, Sobibór, and Treblinka death camps.

The Nazis also built gas chambers at some of the concentration camps that already existed, like Auschwitz (Auschwitz II, called "Birkenau") and Majdanek.

The camps used either carbon monoxide or cyanide gas, with the first gassing occurring late 1941 and the last in August 1944. Cyanide was introduced in Auschwitz-Birkenau extermination camp by the use of Zyklon B, an insecticide, which was thought to be more effective. Carbon monoxide took many minutes to kill, even with the chamber crowded with hundreds of people. Also, in Auschwitz-Birkenau the method of introducing the people to the chambers were more efficient, with large underground rooms disguised as shower rooms, into which the victims walked mostly without resistance and calmly left their clothes and belongings. Because of these reasons, Auschwitz-Birkenau was thought of as the most efficient part of the Nazi killing machine during the Holocaust, and could kill up to 2,000 people a day if a proper schedule was made and followed.

However, most killings took place elsewhere, including other camps and by shooting. Gas was thought to be a better method because it involved fewer people and the ones who helped could be Jews who were replaced gradually. After gassing, the victims were dragged out and quickly burned in large ovens, and the gas chambers cleaned and fanned. After the war and the destruction of the camps, the method was probably disgraced because of the association with the Nazis.

Historians estimate that the Nazis killed around three million people in these camps, mostly by use of gas, but also by shooting and through disease and starvation.  These people included:

In other countries 
The gas chamber has also been used in Lithuania, a small country in the Baltics, before it was annexed (taken over by) the Soviet Union in 1940. After that, Lithuania executed prisoners by the Soviet method of a bullet shot into the back of the head.

References

Torture
Capital punishment