A Centaurian slug was a small parasitic animal which entered a humanoid host through the mouth, eventually traveling through the head to latch onto the brain stem, where it released a toxin forcing the host to answer any and all questions posed.
In 2258, the Romulan Nero used a Centaurian slug in his interrogation of Captain Christopher Pike in a successful attempt to learn the subspace frequencies of Starfleet's border protection grids surrounding Earth. (Star Trek)
Screenwriters Roberto Orci and Alex Kurtzman invented the Centaurian slug as an homage to the Ceti eel from Star Trek II: The Wrath of Khan. They considered having Nero actually use a Ceti eel, but "wanted 'homage' instead of a direct lift." 
In the film's script, the Centaurian slug was described as "the most disgusting creature you've seen" and "a slug with suckers and spikes, covered in a grimy muck." 
A deleted scene from the film Star Trek establishes that Nero got the slugs from Klingon guards on Rura Penthe, who used the creatures to interrogate prisoners, and the scene also included dialogue stating a different mechanism for their effects: when placed into dark spaces, they furiously attempt to burrow their way out. ("Prison Interrogation and Breakout", Star Trek (Special Edition) DVD/Star Trek (Three disc Blu-ray)/Star Trek: The Compendium Blu-ray special features) In the scripted version of the scene, this was revealed to be because they are from a planet bathed in constant sunlight.