It may not be a deep question- but the answer could take a novel or two. Yes, there are a lot of denominations out there and the differences can be vast in some cases- some being so different from each other that they truly qualify as a different faith altogether. I don't think your question can be tackled without delving into history- so I'll give you a link to excerpts from a book that shows things from an Orthodox Christian perspective history and theology wise- as I recall the book delved into basic history plus differences between Orthodox Christianity and others (though I read the entire book and not the excerpts- so I don't know how much of it you'll get here- but it put things in lay terms).
Excerpts from the Orthodox Church by Bishop Kallistos Ware (part 1)
Excerpts from the Orthodox Church by Bishop Kallistos Ware (part 2)
If you enjoy reading history and theology- I've got some reading suggestions for you. This is really only if you're extremely serious about the pursuit of this topic. Check out Jaroslav Pelikan's works- he was a very well respected scholar of Christian history and theology and taught at Yale. He wrote a few well-researched volumes on the subject.
The Christian Tradition: A History of the Development of Doctrine, Vol. 1: The Emergence of the Catholic Tradition (100-600):
Amazon.com: The Christian Tradition: A History of the Development of Doctrine, Vol. 1: The Emergence of the Catholic Tradition (100-600) (9780226653716): Jaroslav Pelikan: Books
The Christian Tradition: A History of the Development of Doctrine, Vol. 2: The Spirit of Eastern Christendom (600-1700):
Amazon.com: The Christian Tradition: A History of the Development of Doctrine, Vol. 2: The Spirit of Eastern Christendom (600-1700) (9780226653730): Jaroslav Pelikan: Books
The Christian Tradition: A History of the Development of Doctrine, Vol. 3: The Growth of Medieval Theology (600-1300):
Amazon.com: The Christian Tradition: A History of the Development of Doctrine, Vol. 3: The Growth of Medieval Theology (600-1300) (9780226653754): Jaroslav Pelikan: Books
The Christian Tradition: A History of the Development of Doctrine, Vol. 4: Reformation of Church and Dogma (1300-1700):
Amazon.com: The Christian Tradition: A History of the Development of Doctrine, Vol. 4: Reformation of Church and Dogma (1300-1700) (9780226653778): Jaroslav Pelikan: Books
The Christian Tradition: A History of the Development of Doctrine, Vol. 5: Christian Doctrine and Modern Culture (since 1700):
Amazon.com: The Christian Tradition: A History of the Development of Doctrine, Vol. 5: Christian Doctrine and Modern Culture (since 1700) (9780226653808): Jaroslav Pelikan: Books
Essentially, you've got a few categories- the non-Chalcedonian Christians (Oriental Orthodox), Orthodox Christianity (from which the Patriarch of the Roman Catholic Church split from the other sees officially in 1054), the Roman Catholic Church, the Reformed denominations that sprung out of the schism with the RCC, and then myriad upon myriad of groups that had their schism upon schism from the Reformation until today. My opinion- the further removed from the Ecumenical Councils one group is (more modern)- the less they tend to have in common with the faith that was Traditioned to the Apostles. We know where the Church is (wherein resides the fullness of the faith), but only God knows who among the heterodox are in His Church.
For example- from an Orthodox perspective- for the Christian, everything can be related back to Communion/Eucharist. Baptism brings one into the Body of Christ. You die to your old self when you put on Christ- and you are to live a new life in Christ's Holy Church- wherein you work out your salvation. When one is baptized into Christ they are part of the body of Christ- the temple of the Holy Spirit- receiving Christ in the Holy Mysteries.
If people are in the Church who don't believe as said Church does- then they really can't expect to take part of the holy mysteries (sacraments). When they engage in practices that are in direct opposition to the Faith that was Traditioned and hold beliefs/views that are not part of the Faith- it is a proclamation that they are refusing to deny themselves for the sake of Christ and are choosing to engage their passions and refusing to repent- they are knowingly excommunicating themselves. The Holy Mysteries are very important to us and we do not risk profaning them- for the protection of everyone. Willfully sinning in this manner is serious business- one risks their soul unless they repent and return to the hospital for what ails us. If someone
outside of the fold doesn't believe as we do- we don't commune them either (even if they claim to be Christian).
Indeed, we see groups who also claim the Christian label who hold nothing in common with us belief-wise (we use some of the same words/terms yet often mean drastically different things), some don't even consider us Christian, and then get upset that we close communion to those who don't share our faith in all ways. The faith is much more to us than a mere intellectual assent to a belief in the Christian God.