It depends, I'm not great at explaining those differences because even Orthodox christians have a hard time explaining them. George was born into a Russian Orthodox family as are most Russians. (yes there are Baptists and others, but most Russians are RO) and he and his parents had trouble explaining it. Like any fossilized church stuck in the dark ages of tradition over the Bible, there are still real believers in the church, but many who are cultural "christians" who rely on being born into, and christened to be saved.
That is NOT always the case. At my fil's funeral the priest in charge was busy trying to make sure he preached the gospel to me because he wasn't sure if I was saved and he relaxed as soon as I explained that yes, Jesus is the Lord of my life, I accepted His death on the Cross for my sins, and believed as did the priest in his Resurrection on the 3rd day. He and his wife love their congregation and work hard to try and get as many of them properly firmly saved as possible.
Closer to Catholic, than Protestant though. Lots of Icons or paintings of the various "saints". Eastern Orthodox view 3D statues as idols, and give a pass for their icons. It's another way they are different from Catholics. Burn a candle or 50 in front of the icon of a saint for extra oomph for your prayers which go to God but some to Mary unfortunately. Yet they look on the statues of various saints and The Lord as idols. A little bit of log in their own eye, before removing the beam out of the Catholics might be in order there but you asked about the differences and that is a biggy. They see Catholics as pretty heathen with the statues though.
The Eastern Orthodox is the overall designation for Orthodox- it includes Greek and Russian Orthodox and some other orthodox groups I think.
They split from western Christianity (then it was almost completely Catholic out here) in 1054 just a few short years before the Normans conquered England. So it's been a while. The Normans played a role because the Pope of Rome wanted help from a Norman invasion so he sent for help from Constantinople and was denied. Bad blood going back a few more centuries. The Patriarch shut down some Catholic outpost churches in his area and the fight was ON.
The Eastern side in Constantinople were cranky about the Pope. They didn't appreciate the power grab by Rome, calling itself the head of the church, and pointed out that they dated back far earlier to the pre Paul missionaries from Jerusalem. The Orthodox church ruled from Constantinople, which remained the centre of Roman civilization while western Rome was fighting off Vandals and Visigoths.
They figured that the upstart Pope aka bishop of Rome had no greater authority than any other arch bishop of a large centre which in Eastern Orthodox circles is called a Metropolitan (not a pope or a bishop). The equivalent to the Pope is called a Patriarch. But those are a bit fuzzy now depending on which country of Orthodoxy we are discussing.
The Pope of that time didn't like that at all so he excommunicated the head of the Eastern Orthodox church (the Patriarch) and The Patriarch returned the favour.
Since both claimed to be the head of the universal church, this got sticky pretty quick as you can imagine.
Then there was the business of celibacy and the priesthood. Priests in the West were required to be celibate with few exceptions and celibacy seems to have been a rather elastic concept considering how many of the Pope's illegitimate children succeeded to the Papacy.
The Eastern Orthodox were quite rightly horrified. And they have always had married priests. In fact you can't be a priest unless you are married, or advance in the church. There are monks who take vows of celibacy, but they are not able to advance in power and rank in the church of the East.
And there is something called the Filoque which simply means that the Easterners view the Holy Spirit as proceeding from the Son of God Jesus, while the Western Catholic and daughter churches view the Holy Spirit as coming from the Father.
And that is the depths of my knowledge on the subject.
They are Christian, they are not heathen, just a branch that ended up suffering horribly when Constantinople fell to the Muslim hordes. The branches of the Orthodox church split into national lines, some going north into Russia others remaining in Greece.
`Edited to add- so I looked it up and there are a bunch, including the Assyrian Orthodox, Ethiopian, Coptic Christians and more. I am having real trouble pasting links in but a pew research article Nov 14, 2017 titled
Q&A: A closer look at Orthodox Christians
has a really good run down on the different branches of Orthodox Christians.