For the duration of the Counter-Reformation, the Papacy introduced a number of 'packages', which were meant to supplement the work done by the likes of the Jesuits, certain individuals and the findings of the Council of Trent.
Strictly, this was called the "Index Librorum Prohibitorum". It was a list of forbidden books published by Pope Paul IV in 1559. It was enforced by the Inquisition and anybody caught with a forbidden book would suffer the consequences. The Index "was an important agent of the Counter-Reformation" (EN Williams).
It was basically a censorship on writing and it was believed that the Index would act as an effective agent against heresy.
As early as 1543, Caraffa., as Inquisitor-General of the Roman Inquisition, had insisted that no book should be published without approval of the Holy Office. He also wanted the Inquisition to hunt out and destroy already published books. Caraffa became Pope Paul IV.
The Index got its final form in 1564.
In 1571, Pope Pius V established a special department (a congregation) of the Index which was in charge of it - it could update and revise the Index.
Added to this was the Index Expurgatorius - a list of books that could be read as long as certain passages were removed.
All the above was a response to the damage done by both Luther and Calvin who presented their ideas in the written form and used books/pamphlets as their main source of marketing their ideas.
This was a feared organisation within the Catholic world. It existed in Spain and Italy. Its success in Spain led to Paul III reviving it in 1542. Six Inquisitor-Generals were appointed (all cardinals) who had power over all clergy and the laity.
Anybody accused by the Inquisition was guilty until they could prove their innocence. The accused were allowed to be tortured - as were witnesses. If you were found guilty, your punishments ranged from execution to the confiscation of your property. The Inquisition’s task was to hunt out heretics.
However, many Catholic rulers did not allow it into their territory because it represented a threat to them. Its work was essentially limited to Spain and Italy where few Protestants were found anyway. In Spain its main purpose was to keep track of those who represented a threat to Philip II; therefore, the Inquisition in Spain was used not so much to remove heresy - there was little need for this - but to hunt out those who might represent a threat to the king - be it financial or political (though the two were both married together). It was the Inquisition in Spain that was sent to the ports to oversee that the merchants there paid their fair share of tax - a task they frequently failed to do. The Spanish Inquisition was also sent to the Spanish Netherlands and to the New World.