The Mass that was celebrated in Latin in this form from the time of the Council of Trent in in 1564 until the Vatican II Council in 1962 when the last Missal in Latin was published before the Novus Ordo or the Mass of Pope Paul VI.  For approximately 400 years, the Tridentine Mass was the Mass celebrated by most Catholics.

The Mass has many differences but also many similarities.  The primary difference is that the priest has his back to the congregation.  This is because he is leading the congregation in prayer.  He also speaks very quietly and, often times, cannot be heard by the faithful.  

It is as Pope Benedict XVI said our current Missal and the MEF are two celebrations of the same rite that although appealing to only a very small percentage of Catholics, it needs to be offered if there are a significant number who request it.

In 2007, the Vatican published a document called Universae Ecclesiae in which they explained that the purpose of the return of the 1962 Missal was to:

1. offer to all the faithful the Roman Liturgy in the Usus Antiquior, considered as a precious treasure to be preserved;
2. effectively guarantee and ensure the use of the forma extraordinaria for all who ask for it, given that the use of the 1962 Roman Liturgy is a faculty generously granted for the good of the faithful and therefore is to be interpreted in a sense favourable to the faithful who are its principal addressees;
3. promote reconciliation at the heart of the Church.