Poison is both a type of Elemental Damage and a collective name of debuffs that deal damage over time. Some poison attacks are stronger than others and some have longer durations than others. Poison is typically associated with color green, taking form of toxins, venoms, acids etc.
There are two super unique monsters who have shot to fame by the strength of their Poison attacks alone. Poison is the main weapon of the Maiden of Anguish, Andariel and the Sand Maggot Queen, Coldworm the Burrower. The latter's poison has been known to leave even the strongest characters at 1 health and almost always the death of the accompanying Hireling, But is only used when she dies and spills her green blood around her. Andariel uses her poison attacks much more often, and in a variety of ways. She has a poison bolt that travels along a straight line that does heavy damage per second if it strikes, or she could spread an arc-like wave of poison towards the player which does less damage than the previous attack but still is very dangerous. She can also inflict poison damage in her melee attacks. Both the above ladies do very high damage with their poison attacks, so it is highly advisable to have Antidote Potions in the Belt before fighting them.
For Battle.net only characters, there is another poison user, much more deadly than the aforementioned ladies of disease, the Demon Queen, Lilith. Lilith can be fought in the Pandemonium Event Battle.net only quest and she packs quite a mean punch with her other worldly poison attacks. They do hefty amounts of damage per second and stay on the character for a very long time. It is imperative that the player fights her with Antidotes on hand. It would also help if you keep some gold ready to resurrect your hireling after the fight.
In Diablo II, most players do not take Poison damage as seriously as Lightning. This may be due to its characteristic of doing damage over time, which can be balanced by using items that give Life Regeneration, and Healing Potions if Antidote Potions are not available. It should be noted that poison will never be the cause of death for the player's hero, stopping at a hit point total of 1. So, no character can be killed by poison damage alone. This can be used as an advantage by PvP players.
When you are poisoned, your Life orb turns green and you slowly lose hit points.
Poison is definitely commonplace in Act II, thanks to the Mummies and their greater cousins alone, although that's not to say that the other acts don't have their share of enemies that make use of this element.
Poison can instantly be cured with an Antidote Potion. Additionally, you can lower the amount of life you lose by increasing you character's Poison Resistance. You can also reduce the duration of the poison with certain magic items and, if you are a Paladin, by using the Cleansing skill.
In Diablo III, poison may be an instant damage as well, but many Poison skills are applying damage over time effects too, or create pools of poison and acid on the ground. Poison damage is almost exclusively used by Witch Doctors, but hardly ever by other classes. Critical Hits from Poison damage may leave the target seething with toxic fumes, or just dissolve it to the bone. Only Witch Doctors roll Poison damage as Smart-Loot.
Andariel's Visage, Stone of Jordan, Lidless Wall, Wormwood and Swamp Land Waders are the items with unconventional bonus to Poison skills damage. This actually leaves this damage type least capable of being boosted, even though it is, in fact, used by just one class. Mara's Kaleidoscope amulet will render the character completely immune to Poison damage (the attacks will actually heal the Nephalem).