Vatican II was a seminal event in the life of the Church that Jesus Christ founded back in 33AD. It modernized the Church, added the vernacular to the Mass, and increased the participation of the laity in the Mass. This Council of the Magesterium back in the 1960's was started by Pope John XXIII, and finished by Pope Paul VI. The 1960's were a very turbulent time in the culture of the world as well, with the sexual revolution, the pill, Viet Nam protests, the Cuban Missile Crisis, Playboy Magazine, the Iron Curtain, the Kennedy assassination, the Cold War, etc. A lot of people in the Church at that time were affected by these external cultural events, including the belief that no one could tell them what to believe, especially when it came to being obedient to the Church. These people started to believe that their conscience would be their guide, rather than the Church, the Pope, or the Bible. As a result, many priests and nuns left their vocation, got married, and went their own way. Sadly, a lot of this laicization was blamed on Vatican II, rather than on the culture shifts of the day.
Some traditional Catholics, like the Society of St. Pius X , broke away from the Vatican, because they disagreed with the ecumenism of Vatican II, and the "new Mass" in the vernacular. They believed that the Latin Tridentine Mass (from the Council of Trent in the 16th Century) was the only valid Mass, and every other Mass was invalid. They took it upon themselves to ordain new priests and bishops without the Vatican's permission, and were summarily excommunicated. Just recently, Pope Benedict XVI lifted their excommunication, and talks are underway to bring them back into communion with The Church. Some people erroneously believe that every Pope after Pope Pius XII is invalid, because they endorsed Vatican II. These folks are called sedevacantists (the seat being vacant).
So we see that Vatican II was a watershed for those on the left who didn't like their vow of obedience and left, and for those on the right who believed that Vatican II itself was not obedient to the Council of Trent and left. Both were wrong. The Vatican II Mass is as valid and holy as any Mass, including the Latin rite. To deny that is to deny the authority of The Church, and makes Jesus into a liar when he said that "the gates of hell would not prevail against My Church."
For sure, there were numerous abuses in local parishes, done in the "spirit of Vatican II" (whatever that is), including horrible music, clown masses, balloon masses, making-it-up-as-you-go-along liturgical abuses, horrible new-church architecture, removal of statues from churches, personal conscience trumping church teaching in the confessional, off-color jokes told from the pulpit and altar, etc. Pope Benedict once said that the implementation of Vatican II was like building a house with no blueprint, with every parish deciding on its own what to do. However, under his leadership, things are starting to change. The Latin Mass can now be said in any parish where the parishioners want it. Before, it was left up to the discretion of the local Bishop when and where it could be said, thus limiting it. On July 7, 2007, Pope Benedict with his Motu Proprio (on his own impulse) "Summorum Pontificum" changed all of that. Pope Benedict is now allowing the , retaining much of their beautiful liturgy intact. He is also working closely with the Orthodox Church, who never really warmed to the new Mass, to reunite with the Vatican after an almost 1000 year split. So the ecumenism of Vatican II is coming to fruition almost 50 years later. Funny how a German priest, Martin Luther, created schisms in the Christian world almost 500 years ago, and now, another German priest, Pope Benedict, is bringing it all back together again.
Some of the best documents in the history of the Catholic Church were written during Vatican II. Below are EWTN's links to all sixteen of them, including Dei Verbum (the Word of God), which delineates the importance of sacred scripture, and shows the world how Catholics read the Bible. If you don't have time to read all 16 of them, at least read Dei Verbum. You'll be glad you did!
THE SIXTEEN DOCUMENTS OF VATICAN II
1. Sacrosanctum concilium, Constitution on the Sacred Liturgy, 1963.
2. Inter Mirifica, Decree On the Means of Social Communication, 1963.
3. Lumen Gentium, Dogmatic Constitution On the Church, 1964.
4. Orientalium Ecclesiarum, Decree On the Catholic Churches of the Eastern Rite,1964.
6. Christus Dominus, Decree Concerning the Pastoral Office of Bishops In the Church, 1965.
7. Perfectae Caritatis, Decree On Renewal of Religious Life, 1965.
10. Nostra Aetate, Declaration On the Relation Of the Church to Non-Christian Religions, 1965.
11. Dei Verbum, Dogmatic Constitution On Divine Revelation, 1965.
12. Apostolicam Actuositatem, Decree On the Apostolate of the Laity, 1965.
13. Dignitatis Humanae, Declaration On Religious Freedom, 1965.
14. Ad Gentes, Decree On the Mission Activity of the Church, 1965.
15. Presbyterorum Ordinis, Decree On the Ministry and Life of Priests, 1965.
16. Gaudium et Spes, Pastoral Constitution On the Church In the Modern World, 1965.