1) Do the Orthodox have a similar position as Catholics do on sin (venial, mortal, etc).
I would have to say that technically every sin is a mortal sin, as any one of them eventually could prevent us from inheriting the Kingdom of Heaven.
Do you also believe, like Catholics, that the only way for sins to be forgiven is through confession and you can't get forgiveness by praying?
Ah... I doubt that's what they themselves believe. The significance of Absolution by a Priest is that it is a tangible sign and therefore assurance of remission of sins. When one is praying by one's self for forgiveness, there is no such tangible sign, and thus no such assurance of absolution. Yes, God certainly could do it. But we don't know whether He would or not.
2) Do Orthodox Christians believe Protestants and/or Catholics will go to hell simply because they aren't Orthodox,
Technically everyone in Heaven will have somehow become an Orthodox Christian. That doesn't mean that those who were Protestants or Romanists in their lifetimes will necessarily be damned. But it does mean that being such puts one's salvation in jeopardy.
or do they believe that these groups are imperfectly in the Church (note: imperfectly)? (Catholics state in their catechism that they believe these groups are in their church "albeit imperfectly" by baptism)
No. We do not have this pseudo-ecclesiology of Rome. You're either in the Church or you're not.
3) What about Orthodox Christians who leave the Church for Protestantism or Catholicism? What shall we say of them? Will they be accepted by God as true Christians? Or will they be denied since they left the Church to try to follow Jesus elsewhere?
They will only be accepted if they recant of their apostasy. The idea that there could be people in Heaven who do not accept the fundamental truths about God is just nonsensical.
4) What do the Orthodox believe happens after death? I understand Orthodox Christians deny purgatory, so what exactly happens?
People, on the basis of the judgment of their lifetimes, experience either a foretaste of bliss or a foretaste of torment. While we do not believe in Purgatory, we do believe that those in the afterlife may be aided by the prayers of the Church. It has even been said a few times that "even the most grievous of sins may be remitted", something that the Romanists would deny.
5) I hear the Orthodox Church is really split up (Greek, Russian, etc). Are these all separate churches? Can these groups take each others communion?
The main Church of Greece and Church of Russia are not "separate churches", though they are separate jurisdictions. They are in communion with each other. And they are also in communion with about 13 other separate jurisdictions.
There are some jurisdictions which have resisted the introduction of the New Calendar. They are called "Old Calendarists". They are not in communion with the above, but they are often in communion with each other.
There are some jurisdictions which rejected the Council of Chalcedon more than 1500 years ago called the Oriental Orthodox Churches. They include the Copts, Syrians, Armenians, Ethiopians, Eritreans, and Indians. They are in communion with each other, but not with the other two. None of these three main groups are in communion with any of the other groups.