If the victim inhales it, he or she will start laughing, then a giant smile with yellow teeth will appear on their face and they will fall down, stiff as a board. There is a cure but you need a sample of the gas to create it. This gas is a powerful neurotoxin that attacks the brain and causes hallucination and muscle spasms.
In other media
Batman: The Animated Series
Joker venom is used frequently in the animated series, and it's spin-offs such as Batman: Mask of the Phantasm. The most popular use of it in the animated series in through a "very special flower" on the Joker's purple suit.
Batman (1989 film)
The Joker uses "Smylex" specifically in gas form, to kill everyone at a celebration in the center of Gotham.
The Dark Knight
What may or may not have been Joker Venom comes out of a grenade (in gas form) in the Manager of Gotham National Bank's mouth.
The Dark Knight (Book)
Shortly after the Joker has first met the crime lords, Sal Maroni hires a private investigator, named Hamlin, to discover who the Joker is. Several weeks later, Hamlin comes back to Maroni completely disheveled and asks if Maroni didn't just imagine Joker, because there is nothing on him, like he came out of nowhere. Then Hamlin begins laughing hysterically and dies in the middle of a restaurant. Maroni finds out Hamlin had been poisoned by some herb from out in "China, Tibet, Korea, one of those places" (he's indifferent to remembering where exactly). It's also implied in this scene that the Joker venom was derived from the same plant that Scarecrow's Fear Toxin was made from, as the Toxin was derived from a Blue Poppy in Tibet.