A Poisoner can target a player to kill them. However, this kill will take place at the end of the next Night. Thus, the poisoned player can get one more Day and Night of actions out. The advantage to this is that a kill via poison cannot be stopped, except by the fairly rare Poison Doctor.
Poisoner has been seen as a substitute for Vigilante; however, Poisoner is much more commonly an anti-Town role who uses the poison as their implementation of the factional kill.
Exactly when the poison kills a player is not standardized. Some moderators will cause the poison to take effect at the end of the next Day, and some moderators will cause the poison to take effect a few posts into the next Day. The power of the Poisoner will vary nontrivially based on which time of death is chosen - the longer the wait, the more actions the poisoned players can perform.
Some moderators allow ordinary Doctors to stop poison kills; however, this usually makes the poison-based kill almost strictly inferior to simply killing the victim outright.
In some setups, players will be aware that they have been poisoned. It's possibly more common for the poisoning to be kept secret until it takes effect, however.
Use and Power
Poisoner provides an interesting trade-off between bypassing the potential for a player to be protected at Night and allowing the victim to perform more actions before dying. It's used in setups when this may become a factor, or simply for spice.
Note that the role is not quite strictly inferior to Strongman; it's possible (if unlikely) that town will end up wasting an elimination on the player that the Poisoner tried to kill (whereas they could not have made this mistake if the player had already died). If there are an even number of living players going into night, a kill delayed until the end of the following Day (the optimal time for it, from the killer's point of view) will give Town the minimum possible information from the Night, whilst still not allowing the target to perform any Night actions nor changing the time at which ELo occurs; as such, a Poison kill can be significantly stronger than a regular or even Strongman kill in some circumstances. This timing is, however, notably worse in the less common situation where there are an even number of players during the Day (and thus an odd number of players going into Night, after the elimination).