I'll try to list here some basic differences between RCC & Orthodoxy, although it's a huge topic that would require a much more in depth response to be fair.
We believe that we must keep the faith which Christ taught to the Apostles, which the Apostles passed on to their successors, which has been passed down from generation to generation until today, without change. We don't think that we can figure out stuff that wasn't revealed before, that we come to understand points of faith that we didn't before. The expressions of the faith, ie the doctrines did develop over time, that is, the way the faith is explained progressed and was refined, but the faith itself remaines unchanged. Catholics on the other hand belief in doctrinal progression, that the understanding of the faith progresses over time. An example of the is the title of Coredemptrix for St. Mary, which now Catholics are free to agree with or disagree with, but may be defined by the Pope as the Immaculate Conception was, in which case Catholics will suddenly be bound to believe it, this kind of change is foreign to our understanding of the Gospel.
Catholics believe that the Pope of Rome is the shephard of all Christians, the Vicar of Christ, and protected by the Holy Spirit from defining any doctrinal error. We believe that Christ needs no vicar, that He Himself is the head of the Church, practically as well as spiritually, and that all the patriarchs are equal, and all are fallible.
The Divine Liturgies in Orthodoxy, what the Catholics call the Mass, are the Liturgies written by the Apostles as they spread Christianity around the world. We do not believe that you can just go and write a new Liturgy as the Catholics did in the '60's with their new Mass, as we believe that Christ taught the Apostles how to write them, and while saints have added prayers to the Apostolic Liturgies as appropriate to changing circumstances in the Church, the essential Liturgies remain Apostolic, and always will in Orthodoxy.
We disagree with the Imaculate Conception, we believe that Christ took on our nature, and became like us in all things but sin, and that He took that nature from St. Mary, not that He purified her first and took on a nature different from ours, already purified. We believe that He took on flesh like ours, and defeated the death in it since He is life Himself, defeating death and sin by His death, so that we can all have life in Him.
We believe the ancient doctrines about St. Mary, that she fell asleep (died), was ressurected as we all will be at the general ressurection, and was assumed bodily into heaven. Catholics questioned these, and the assumption was later defined again and must be believed by Catholics, although Catholics are still free to believe either way on the Dormition. We accept the faith passed to us by our fathers without this kind of questioning and going back and forth.
We Chrismate infants immediatly after Baptism, and give them Comunion immediatly, and regularly. We don't hold back God's saving grace from our children thinking that we must do the work of understanding before His grace is effective, Christ said to let the little children come to Him and we do.
We believe that the Holy Spirit comes down through the prayers of the people and literally changes the bread and wine to the Body and Blood of Christ, that it is no more bread or wine, but the Body and Blood. This sums up all our believes on the change, we do not try to go further than this as do the Catholics, but leave it as a Mystery, simply.
We do not distinguish between mortal and venial sins and think that we do not need to confess "small" sins, we must confess them all, and strive to meet the commandment "be ye perfect as your Father in Heaven is perfect". There's no penance like 10 hail Mary's after confession since it isn't about guilt but about reconciling a sinner to God, coming before God, confessing our sins, and being accepted back by Him. It is His work, not ours.
We give the Annointing of the sick to all who are seriously ill, not just the dying as has been the Catholic practice.
We do not prevent married men from becoming priests, although once a priest a man can never marry, as is the ancient practice.
We do not have annulments like the Catholics do, we allow divorce in the case of adultury only, as Christ commanded. Marriage is indisolvable, the two literally become one by the coming down of the Holy Spirit. When the marriage is consumated it is inseprable, and nothing but an abuse of sex later can split what God has joined in two. We believe the priest confres the Sacrament of marriage like all the other sacraments, not the couple.
We do not allow marriage outside of Christianity as the Catholics do, St. Paul was talking about two unbelievers & one becoming Christian, he wasn't giving permishion for a Christian to marry an unbeliever.
We don't speculate about the salvation of those outside the Church, and we don't presume God's grace by saying that they can be saved, or judge in His place by saying that they are ******. We only know what God has told us we must do to be saved, and what we must bring to others so they may be saved, and do our best to do so, we leave the judgment to God.