[Letum]

All you need for chlorine gas is salt water and a electrical socket or battery. Stick the terminals into the salt water and chlorine will quickly start bubbling up.
Then you just have to pump it into a air compressor after separating the hydrogen.

Chlorine reacts with water in your lungs and eyes to form hydrochloric acid which burns the lung lining causing bleeding in the lungs. Heavy exposure will cause the lungs to fill with liquid and the victim drowns.
Chlorine is most effective as a weapon in a dry, enclosed environment with little wind.

Chlorine was used in both world wars, but mainly the first. It was quickly abandoned in favor of more effective gases.

It is not very deadly compared to more advanced chemical weapons, but can cause blindness and breathing problems.

You would need a lot of chlorine to cause more than a few deaths in a urban area.
*edit* By "a lot" I mean more than quarter of a ton of gas.

When the Germans used over 150tonnes of chlorine there where only ~150 deaths. Obviously the death rate would be somwhat higher in a urban setting.

It's easy to defend against chlorine, just cover your mouth with a wet cloth and the chlorine forms acid on the cloth and not in your lungs.

You can see and smell chlorine. This alone can make it an effective terror weapon.