Sarin, also known by its NATO code of GB and with the chemical name O-isopropyl methylphosphonofluoridate, is a lethal, nonpersistent chemical weapon, of the nerve agent family. It was first synthesized by Germany in World War II, but was not produced at the time as its manufacture was more difficult than that of Tabun (GA). The name reportedly came from the names of its developers: Schraeder, Ambrose, Rudriger, and van der Linde. 
After the war, it was stockpiled by the Soviet Union, United States, and other countries, although banned by the 1972 Chemical Weapons Convention and has been subject to national destruction programs. It was stockpiled, and used in the Iran-Iraq War, by Iraq, and probably was still in the Iraqi stockpile at the time of the Gulf War, but most appears to have been destroyed by the Iraq War in 2003.
While there are several ways to make sarin, the usual final synthesis generally combines the precursor "difluor" (DF or methylphosphonyldifluoride), with isopropanol and some reaction promoting reagent, such as an organic base. Synthesis of DF, which has no other applications, is decidedly nontrivial and itself involves controlled chemicals, some with dual-use.
While liquid sarin is exceptionally dangerous and a small amount can penetrate unbroken skin and kill, the primary goal of delivery systems is at least to aerosolize the liquid, and, when possible, have it evaporate into a gas. The derivative cyclosarin is less volatile and is intended to contaminate areas as a persistent liquid.
National militaries stockpiled it in aircraft spray tanks, bombs, artillery shells and missile warheads. Unless the sarin was quite pure, and any excess, inflammable, isopropanol was removed, it could catch fire and degrade if dispersion was by an explosive warhead burster. Spray tanks, which were used by Iraq against civilians, are more efficient and more likely to work, although far more difficult to use against an enemy with air defenses. Iraq's were on helicopters.
Perhaps the most common tactical delivery system was the 155mm howitzer. Warheads were developed for tactical ballistic missiles including the SS-1 SCUD, but, in the case of the Iraqi implementation, the combination of a burster warhead and impure sarin gave a significant chance the sarin would burn. The US had a dedicated 115mm unguided rocket system for delivering chemical weapons.
Impure sarin also degrades fairly rapidly, and that was one of the reasons for developing binary munitions, in which the final two chemicals were mixed in an artillery shell or bomb, just before release. Iraq used a field technique in which a soldier, wearing personal protective equipment, would mix the final two liquids just before they were used to fill casings about to be launched. The reason that binary munitions were of interest to powers that could produce the stable agent was that the munitions were far less dangerous in storage.
It was used by the terrorist group, Aum Shinryo, in a 1995 attack in the Tokyo subway, and in earlier, smaller attacks in 1994, not immediately recognized at the time. While the 1994 attack caused deaths and used a spray tank, the sarin was of low purity. Eight people died in the subway attack, but the death toll would have been far worse had it been even more slightly more efficiently dispersed. Aum operatives simply punctured plastic bags of the liquid chemical and relied on evaporation.
What appears to have been a binary GB artillery shell was used in an improvised explosive device in Iraq in 2004, although there were no significant toxic effects. The very specific and extreme conditions required for binary mixing, such as the high gravitational load of artillery firing to break the separator between the two precursors, and the spinning of an artillery shell required to mix them, did not take place. The Iraq Survey Group concluded this was a pre-1991 munition.
Sarin and its major precursors are explicitly listed in the Chemical Weapons Convention/Schedules and subject to international export controls for those that do not have extensive civilian dual-use, such as isopropanol. National controls vary; a BBC news investigation showed that in 2003, it was possible to buy the precursors within the UK. 
- Sarin, Nuclear Threat Initiative
- Yasuo Seto (1 June 2001), The Sarin Gas Attack in Japan and the Related Forensic Investigation
- Iraq Survey Group, Iraq’s Chemical Warfare Program. Annex F, Detailed Preliminary Assessment of Chemical Weapons Findings, Iraq Survey Group Final Report
- "Deadly Sarin 'easily available in UK'", BBC News, 30 May 2003