In the 16th Century the Catholic Church was in need of personal reform. Many Catholics had not undergone the Second Conversion, as spiritual authors call it. The clergy were in the main sinners to say the least. Although Huss and Wycliffe had planted the seeds in the century before, they had not founded a lasting movement. That honor fell to Martin Luther who began by nailing 95 theses to the church door of Wittenberg over five centuries ago. This is considered the founding of the Protestant movement, which continued on with others, such as Calvin and Zwingli, who edited Luther’s doctrine. From this many other sects have flowed, which are called Protestant, because they protest against Jesus Christ’s Church.
Notice I am calling this the Protestant Revolt, not the Protestant Reformation as many call it. You cannot reform what does not yet exist. Protestantism could have a reformation today, because it exists. In fact there have been attempts to unite Protestantism into one sect, but today this appears impossible.
Now the Catholic Church through the Council of Trent went through a reformation, because much of this Council was devoted to enforcing Divine and Church Law and reforming the clergy. Some call this the Counter Reformation, which it cannot be. True the Protestant Revolt brought attention to the problem, but the Catholic Reformation is just that a movement to return to living by the Gospel.