What gave the Spanish Inquisition its notoriety was its ethnic character: it was set up primarily to root out false conversions among the forcibly converted populations of conquered Spanish territories. The Christian Reconquista of Muslim Spain was completed in 1492 by Castile's defeat of the relatively tolerant Muslim state Granada and the resulting forcible conversions. Many in the target populations only pretended to convert, and the Spanish Inquisition was the response of the aristocracy and clergy to deal with the diversity that had contributed to the Spanish mestizaje or mixture of races and cultures. Transported to the new world colonies of Spain, the Inquisition dealt with even greater and thus more threatening diversity. "For the love of God, any kind of torture or murder was not merely acceptable, it was a moral act."
The Spanish Inquisition gave Spain a head start on other countries that would end up predominantly Catholic when the Reformation came around, giving them an established theocratic-bureaucratic apparatus with which to repress Protestantism.
Spanish Inquisition in popular culture
The "Spanish Inquisition" sketch is one of the more famous creations of the British comedy troupe Monty Python's Flying Circus. Over the course of the episode it is in, at various times people being grilled over some issue declaim "I didn't expect the Spanish Inquisition!". What then ensues in each case is fine Pythonian hilarity.
You can watch it if you want.
- Less Wrong - Guardians of the Truth - The difference between how the Inquisition plays with the "truth" and science plays with the "truth".