I asked a friend about Orthodoxy, specifically the difference between it and Catholicism and this is what I got.
There is much that could be said about this topic. There are several differences between Catholicism and Orthodoxy. Some are superficial or based on differences in regional and national cultures. Others are significant religious disagreements. In my opinion, there are two differences between Catholicism and Orthodoxy which are most important: the filioque and papal authority.
The filioque is a statement in the Nicene Creed. (http://en.wikipedia.org/wiki/Filioque) As you know, the Nicene Creed is a summary of Christian beliefs that has been used by the Church in most of its jurisdictions since it was written in 325 A.D. It was written at a council of all the Church's bishops, and it was amended to include discussion of the Holy Spirit by a later church council. Otherwise, the Creed was not to be changed; it was to remain a standard for Church teachings and use. Later, however, starting in Spain and then in Germany, some regional churches added a word to the Creed: "filioque." It is Latin and it means "and from the Son." It is a change in the teaching about the Holy Spirit. The original teaching is that the Holy Spirit proceeds from the Father -- alone. (This comes from John 15:26) The change says that the Holy Spirit comes from the Father and the Son (This comes from John 20:22 and other verses.) The filioque is not only about theology; it is also about spirituality. The filioque tends to de-emphasize the Holy Spirit as a Person of the Trinity -- it tends to make the Holy Spirit into a kind of by-product of Divine Love between the Father and the Son-- and therefore the filioque tends to deemphasize devotion to the Holy Spirit as well. Historically, the filioque was accepted by the Catholic Church eventually. The Orthodox Church, however, consistently has rejected it. Numerous attempts have been made over time to resolve the dispute theologically, but they have not succeeded because the issue is to solely about theology.
The dispute is also --and perhaps mostly-- about papal authority. The filioque was ultimately approved in the Catholic Church by the papacy acting by itself. The Bishop of Rome has long been recognized as the first among equals of bishops in the Church. The disagreement is over how much authority that status has. The Catholic Church regards the Papacy as the supreme authority in the Church after Christ Himself. Therefore, the Catholic Church sees nothing wrong with the papacy changing the language of the Creed. The Orthodox Church, however, views the papacy more as a figurehead position, and it views the Tradition of the Church (which includes the Bible and certain church councils) as the highest authority in the Church after Christ Himself). One result is in organization. The Catholic Church is centrally organized around the Bishop of Rome, but the Orthodox Church is locally and nationally organized around local and national bishops. Another result is about beliefs and practices. The Catholic Church can (and has) changed its teachings (such as the filioque, Marian dogmas, papal infallibility) and its practices (for example, the Mass and devotions) according to the orders of the papacy; but the Orthodox Church is very conservative and reluctant to change except by conciliar agreement among Orthodox bishops and people.
To summarize: The main, underlying differences between the Catholic Church and the Orthodox Church are about theology and authority in the Church. The Catholic Church has accepted the filioque on the basis of papal authority and teaching. The Orthodox Church has rejected the filioque as being against the consensus of Orthodox people and tradition. There are many surface differences too, in custom and culture. Yet, the basic difference is in different ways of understanding God and our relationship to Him.
