The Joker has used Joker Venom on dozens, perhaps hundreds, of occasions, often as a way of punishing people who have displeased him. Just as much, however, he has used it on people who were simply in the wrong place at the wrong time. In its very first appearance, it appeared as both a liquid and as a gas.
Since then, the Joker has come up with a variety of creative ways to store it. In liquid form, it is typically smeared onto blowdarts and other bladed weapons, or otherwise mixed into a victim's food. In at least one comic (The Man Who Laughs), the Joker dumped gallons upon gallons of the venom into the Gotham reservoir.
In gas form, the venom is most famous for being stored inside the fake flower on the lapel of the Joker's tuxedo, to be deployed as a surprise weapon in close combat. In its first appearance, however, it was in fact shot from a gas gun. A recurring crime for the Joker is to attempt to kill large amounts of people with the gas version of the venom, which he often attempts by storing them in balloons, or otherwise deploying them from hidden nozzles.
Most of the time, the Joker is depicted as immune to both the liquid and gas varieties, often explained as "the product of years of dedicated abuse".
Once inhaled, succed, or injected into the bloodstream, the poison often takes hold immediately. Most of the time, the victim is overcome with violent spasms of laughter which they have no control over, culminating in death by suffocation. Post-mortem, the muscles of the victim's mouth are stretched into a wide, teeth-bearing smile in tribute to the Joker himself.
According to the S.T.A.R. Labs 1993 Annual Report, Joker Venom is "a hellish mixture of hydrogen cyanide and Strychonide, the toxin causes immediate cessation of heart and brain functions."
Various writers and artists have exaggerated the effects of Joker Venom; originally, it killed the victim without inducing any laughter, and did not affect the body in any way besides stretching their mouth into a smile. Since then, it has been depicted as making the victim's face much more similar to that of the Joker, including turning their hair green, making their eyes bulge, and draining their skin of all color, turning it chalk-white.
The effects of the venom on the victim's mind has also been expanded upon; in the cases where the venom does not kill its victim immediately, it has been shown to severely warp their mind, creating fits of Joker-like insanity. Most prominently, this was shown in the "Joker: Last Laugh" mini-series, where the Joker infected every prisoner inside the super-prison known as "The Slab" with it, turning them hopelessly insane.
Much like its owner, how exactly Joker Venom was created has varied over the years. In some comics (such as Detective Comics #168 and The Killing Joke), its creation is hinted to be tied to the Joker's origin as a worker at a chemical plant.
The standalone story "Images" features a decidedly different version of the venom's creation; in this version, the Joker has an autistic savant cousin by the name of Melvin, a genius in chemical engineering. Joker convinces Melvin to create the toxin as a way of "making people laugh", and is told that, in exchange, Joker will make him "handsome" (ironically, Melvin is already extremely handsome; he is unaware of this due to years of his abusive mother berating him as "ugly"). 
As stated above, the venom made its debut in Batman #1, in both the stories "The Joker" and "The Joker Returns". In the former, the Joker uses a special variety of it to kill millionaire Henry Claridge, which takes twenty-four hours to kill (without any side effects before it kills the victim). He also uses the regular version of the venom to kill millionaire Jay Wilde and Judge Drake. At two points in the story, the Joker also utilizes a gas version of the venom, which does not kill, but instead paralyzes the victim with the same grin as the lethal version.
Detective Comics #332 features a very different version of the venom, this time identified as the pollen of a plant known as the "Loco Weed" which affects the central nervous system. This "venom" only makes people laugh uncontrollably, and wears off after a few hours (as befitting the more harmless Silver Age Joker). Batman and Robin are able to make themselves immune to the pollen by swallowing antihistamine pills.
In Detective Comics #475, Joker creates a special variety of the venom that instead affects fish, freezing their mouths into smiles much like his own (and besides turning their lips red and their faces white). He then attempts to patent these fish, and when refused, furiously attacks several bureaucrats from the patent office. The first of these bureaucrats dies from another variation of the venom, created from two binary compounds that are harmless when separate, but form the poison when they make contact with one another.
In Detective Comics #476, the direct follow-up to #475, an effect of the venom used on the fish is seen: a cat, upon holding the fish in its mouth and swallowing some of it, is driven mad and attacks its owner, one of the bureaucrats. Its fangs, contaminated with the venom, sink into the bureaucrat, killing him.
During the "A Death in the Family" story arc, the Joker manages to mix the venom out of things in a janitor's closet at Arkham Asylum, and is able to easily escape this way. Later on, he mentions his plans to replace a shipment of medical supplies with the venom, condemning hundreds, if not thousands, of ill people to death.
Batman Adventures Annual #1 showed yet another version of the venom, this time in liquid form and coated onto a custom-made dollar bill featuring the Joker's face. This venom is activated by sweat, and seeps directly into the victim's pores, as an unfortunate donut boy found out.
In Batman: Dark Victory, the Joker laces his henchmen's masks (shaped exactly like his own face) with the venom. His reasons for doing this are not clear, but it is most likely to prevent them from giving information to the police (or Batman) in case they are caught, or otherwise another display of his twisted sense of humor.
The Joker: Last Laugh mini-series features a version of the venom that kills extremely slowly, but in the meantime drives its victim completely insane while changing their physical appearance to resemble that of the Joker's. The Joker infects the super-powered prisoners inside "The Slab" with the venom and unleashes them onto the world, hoping to die with the biggest performance of his life (as he had been tricked into thinking he had a fatal brain tumor). Harley Quinn, furious at the Joker for attempting to rape her, is convinced to create an antidote for the venom.
The Venom, re-named "Smilex", was featured prominently in the 1989 film Batman. Here, Joker is hinted to have created it all by himself, as he had previously worked at the chemical plant where he turned into the Joker. Upon taking over his former boss Grissom's criminal empire, the Joker holds the city at his mercy by chemically altering everyday hygiene products to become components for the toxin, causing people to become poisoned by the Venom if they use a certain combination of products.
After Batman destroys the factory where he is manufacturing the products, Joker tries once again to kill the city with Smilex - this time, by leading a parade with balloons filled with the gas form of the toxin, which he intends to pop when enough people have gathered. To lure people to the parade, he begins to throw thousands of dollars in cash onto the streets, but before he can release the Smilex, Batman intervenes, towing the balloons away with the Batwing. Here, the Joker shows no immunity to it - he can be seen putting on a gas mask during the parade.
In the DC animated universe, Joker Venom was first utilized in Batman: The Animated Series, in the episode "Last Laugh". Here, it is presented as a gas, created from mixing a number of chemicals and garbage together. When inhaled, the gas is not immediately lethal (though death is hinted to be the eventual result). Instead, it causes the victim to break out in fits of laughter and lose all rationality, something that the Joker uses as a distraction while he and his men loot Gotham. Unlike later presentations, Joker shows no immunity to the Venom here; he can be seen donning a gas mask similar to a diver's helmet.
Later on, Joker develops several variants of the toxin, including the fish-affecting and binary compound strains seen in the "Laughing Fish" storyline from the comics (appropriately enough used in the animated adaptation of the storyline). Much like in the comics, he also stores the gas inside the flower on his lapel, to be used as a secret weapon.
The venom was eventually depicted as being lethal, with Joker's former partner Sal Valestra being the first victim depicted as being killed by it (in the theatrical film Batman: Mask of the Phantasm). In Batman Beyond: Return of the Joker, it is also used to kill Bonk, one of the Joker's own men (though this was only in the censored version).
Due to Broadcast Standards and Practices rules about onscreen death in shows aimed at pre-teens, Batman was depicted as having an antidote to the Venom on hand, as to prevent its early victims from dying.
The 2004 animated series The Batman, being lighter in tone than Batman: The Animated Series in general, presented a more "kid-friendly" version of the Venom. Here, it is referred to as both "laughing gas" and "Joker gas", and simply places victims into a coma while freezing their mouths into enormous smiles. The Joker's first crime involved the Venom prominently, as he intended to burst a hot air balloon filled with the Venom on a statue in the heart of Gotham, exposing the entire city to the gas.
A lethal variant showed up later on, in the episode "The Laughing Bat". Here, it is injected into Batman via a snake-like needle. This version of the venom slowly kills the opponent in twenty-four hours, and in the meantime, causes them to break out in uncontrollable fits of laughter. By the end of the episode, Batman is able to make an antidote to this venom as well.