There are hundreds of threads on this subject a simple research will bring much answers, but I will give some explanation.
Facing liturgical “east” is a very old tradition and for most of the Church’s life the mass has been said facing the “east”. Now some Churches were not built so that the priest could literally face the east, so this is where we get the liturgical east. With this also comes the misconceptions that the priest has his back towards the people, this is a big error in thinking. He is leading the people and their prayers towards God and as one with the community we all face towards the Lord. Just as the sun comes from the East, so too will Jesus Christ come from the East in the end of times, this was always the belief of the early Christians hence why they faced the east for prayer. There are some “evidence” that some masses were said where the priest did it facing the people properly called versus populum, but this practice was not widely used. This practice did a resurgence after the council of Vatican II, even though the documents do not mention anything on this practice it became very popular in the early 70s, its origins after the council are still debatable, but it’s a practice that caught on very quickly and in my opinion very unfortunate. There are many pros for ad orientum (facing east) like, we are all facing the same direction as one, the priest does not have to worry about putting up a “performance”, the act alone shows more reverence, and more. You can google, youtube, and search this subject and there is a lot of good info. The Extra Ordinary Mass (Tridentine Mass) is done only in ad orientum. The Novus Ordo or Ordinary Form can be done in both manners.