Many events in history are important to study, and despite being the subject of a popular Monty Python sketch, the Spanish Inquisition is no exception. In order to understand the Spanish Inquisition, however, you must first have an understanding of the time in which it occurred.
The Spanish Inquisition took place from 1478-1834. It was started by King Ferdinand II and Queen Isabella of Spain, and the Pope, who at that time was Sixtus IV. The King and Queen wanted the country of Spain to be unified under a single religion, and they chose Catholicism to be that religion. In order to do this, Jews and other non-Catholics were persecuted and eventually forced to leave Spain or to convert under the Alhambra Decree in 1492.
One of the most important things you must know to understand the Spanish Inquisition is that people were not asked to convert to Catholicism or encouraged to do so. Anyone who remained in Spain after 1492 and either was not Catholic or was suspected of feigning a conversion was labeled a heretic and brought to trial.
To understand the Spanish Inquisition trials, you may want to compare them to the Salem witch trials in America. There were many similarities between the two. For instance, a person brought to trial in the Spanish Inquisition was on his or her own, as anyone who bore witness for him or her would be accused of being a heretic as well. Also, those who confessed to being heretics, whether they truly were or not, were generally let off with a much lighter punishment, but in return they were expected to give names of other heretics.
If an accused heretic refused to confess, he or she would be tortured to extract the confession. The skill of the Inquisitors, as those who carried out such torture were known, was great. One thing you may find perplexing when you try to understand the Spanish Inquisition is why a person would confess to things he or she did not really do. The Inquisitors were extremely good at extracting confessions via questioning, as they could quote many parts of the Bible, and often simply confused an accused heretic into a confession. If questioning didn't work, physical torture almost always did.
The most important reason to understand the Spanish Inquisition is so that nothing like it happens again. Many people were falsely accused, tortured, and killed simply for being different, which should never be allowed.