A taser is an electroshock weapon, used by police forces of a variety of races, including Rakelli, Romulans, as well as Lyrans.
Romulans, especially the Tal Shiar, used tasers as torture instruments in order to extract information from suspects. (RIS Bouteina: "Kingdom Hospital")
It could also be used to stun rowdy people, as it was used, namely by S'arah, during a Battlefest tournament to restore order to a starbase where the tournament is held. (RIS Bouteina: "Toy Dream")
Lyrans used tasers as torture instruments by military police to carry out court martial sentences. (Star Trek: The Stoneship Files: "Cruchitude Dump")