What are chemical weapons, who has them, are they in compliance with international law, and what can be done to protect people from such weapons?

There are many types of chemicals that have been designated as chemical weapons by the international body established by the UN to prevent the use of chemical weapons. It is called the Organization for the Prohibition of Chemical Weapons (OPCW) and is headquartered in the Netherlands.  Only seven nations are not parties to the OPCW.  Israel and Myanmar (formerly Burma) signed but failed to ratify the 1993 agreement.  Five other nations failed to sign it:  Syria, South Sudan, North Korea, Angola and Egypt.

Nations who are parties to the Chemical Weapons Convention/OPCW agreed to destroy all their chemical weapons by May 2012, but most have failed to meet that deadline, including the United States.  Furthermore, it is by no means certain that all nations possessing chemical weapons declared them, so information on existing stocks of such weapons is incomplete.

Had all 189 nation members of the OPCW complied with the terms of the Chemical Weapons Convention's required destruction of chemical weapons, there would be many fewer such weapons available that could be transferred and used.  The US still maintains large stockpiles of sarin and other chemical weapons.  This US Army map shows you where it was and where it still is located.

So when Obama says that we know Syria's Assad has chemical weapons, Assad could be saying the same thing about us!

From an OPCW report:

Chemical Weapons Destruction Under Way

The 7 States Parties (A State Party, Albania, India, Iraq, the Libyan Arab Jamahiriya, the Russian Federation, and the United States of America) which have declared chemical weapons must destroy 8.67 million items, including munitions and containers containing in total, 71,196 metric tonnes of extremely toxic chemical agents. Albania, India and a third country have completed destruction. By comparison, a tiny drop of a nerve agent, no larger than the head of a pin, can kill an adult human being within minutes after exposure.The OPCW verifies that the destruction process is irreversible. At the same time, States Parties in the process of destroying chemical weapons are obliged to place the highest priority on the safety of people and on protecting the environment.

55,939, or 78.57%, of the world's declared stockpile of 71,196 metric tonnes of chemical agent have been verifiably destroyed. (As at 28/02/2013)

3.95, or 45.56%, of the 8.67 million chemical munitions and containers covered by the CWC have been verifiably destroyed. (As at 28/02/2013)

The OPCW has developed an internationally unique, peer-reviewed, and certified analytical database, containing information on over 3,400 chemical weapons-related compounds. This database is essential for on-site verification activities of OPCW inspection teams, and is also made available to States Parties.

So, although although all 189 member nations agreed to destroy all chemical weapons in their possession by March 2012, less than half of CW weapons in munitions (those ready for immediate delivery to targets) had been destroyed by the deadline, including those possessed by the United States.

OPCW and CDC have posted instructions on how to protect oneself indoors from a chemical weapons attack.

UPDATE Sept. 26:  From the Anniston, Alabama Star:  A detailed history of the US chemical weapons stockpile, leaks and destruction. An excerpt:

For years, workers in chemical suits dealt directly with the chemical weapons, taking them out of storage, moving them to destruction facilities where they could be destroyed or neutralized through a chemical process. 
It was, by all accounts, dangerous work. The chemical munitions were indeed munitions, capable of exploding. The M55 chemical rockets that made up much of the arsenal, both in Anniston and elsewhere, were known to be leaky. News accounts from the 1990s and onward refer to the dangers of the aging rockets in the stockpile. But a history written by officials at Edgewood arsenal states that the rockets first started leaking in 1966, shortly after they were built. Abrams also confirmed that the rockets began leaking a few years after they were built...
Monteverde said the design of the weapons has made destruction a difficult problem at every facility.
"They were never designed to be disassembled," he said. "It was always thought, at the time of their manufacture, that they were designed to be used."