A prisoner was a term for an individual incarcerated in a prison or a similar facility.
An individual would become a prisoner in order to be prevented from continuing actions that the imprisoning body deemed unacceptable. Unacceptable behaviors that resulted in imprisonment varied depending on culture and local laws; common reasons for incarceration ranged from political activity to terrorism, from major to minor crimes. In addition, prisoners of war were individuals who were imprisoned by their enemies as a result of their opposing affiliation.
The Jedi were known to have kept prisoners, but did not believe in executing their prisoners. Volfe Karkko, an Anzati Dark Jedi, experienced typical Jedi imprisonment procedures when he was held in a stasis field, a device that only a Jedi could be capable of opening. In this suspended animation, Karkko's mind was left conscious to contemplate the atrocities he had committed, and his lightsaber was left with him to serve as a warning of the consequences of arrogance.
The Sith commonly tortured their prisoners for sport, after interrogating them for any useful information they might have. The Academy on Korriban at the time of the Jedi Civil War, under the control of Sith Lords Revan and Malak's Empire, often kept their own store of captive Republic soldiers on whom they practiced their skills with a lightsaber as well as their powers of the dark side. Ironically the same Revan was put in stasis by the Sith Emperor for up to 300 years.
The anti-alien Galactic Empire often used their prisoners, who were often a number of different sentient species, for slave labor on a number of planets, such as Kessel and Artus Prime and for construction projects such as the Death Star.