The Council of Trent was the centre of the Catholic Reformation
movement in the 16th century. It sat in three main sessions:
The council was not widely attended at the first session (only 34 bishops were present) but by the final session 200+ attended. Due to a series of wars that were being fought in Italy primarily between the Spanish and the Italians mostly supported by the Papal States the first two sessions were mostly attended only by Italians, however the final session was more dominated by Spanish clergy, the wars being over by that time. In addition the only session attended by French bishops was the third one as Francis I banned any French bishops from attending as he was at war with Charles V.
There was much debate over the location of the General Council. The Papacy wanted it to be held in the Papal States, the French wanted it in France and the Holy Roman Emperor wanted it in his Empire. The location of Trent was reached as a compromise as it was in Italy while still strictly being in the boundaries of the Holy Roman Empire, it was also German speaking which pleased Charles V.
The first session focussed mostly on doctrinal definition which was badly needed to bring the Roman Catholic Church into line with the extremely accurate and well defined doctrine of the Lutheran and protestant churches. This was not as Charles V, Holy Roman Emperor wanted it though. He wanted reform of the abuses within the church so that he could reunite his Empire. The first session ended when Paul III attempted to move the council to Bologna well inside Papal territory. His excuse was that there was cholera outbreak in Trent, however he had hoped to move it unnoticed as Charles V was busy fighting again. Charles’ representatives however refused to move and this bought the effective work of the first session to an end.
These abuses were dealt with in the second session though. However, the large protestant presence during the second session and the continued war in various parts of Europe through out it made it less successful. This lack of success is partly why the next Pope elected was Paul IV a hardliner who attempted to reform the Church himself.
The decrees of the Council are detailed in Decrees of the Council of Trent.