So what would you describe as the largest difference?

I think, biggest difference is, that Islam is the only religion, where we have one God.

Judaism and Christianity also have this one God, not several ones. People of different faith often misunderstand the Trinity in Christianity (Holy Spirit, father, son). Those are not several gods!

Um, nope.

The Holy Trinity may confuse some people and make them think that Chirstians serve three gods, or maybe one sometimes, or something silly like that. But it works like this: Jesus, The Holy Spirit, and God the Father (Body, Spirit, and Mind) are all one piece of a greater whole. Think of each piece as a body part: the arm isn't a leg, and the leg isn't a head; they are all indeed separate and distinct parts, but together they make up a complete body that is one.

In Christianity you are saying that Jesus is a God having become human.
And in Christianity you pray to Jesus, as if he is a God. Or, am I wrong?
Christianity Wiki
As I live in Christianity country, I ask people about this a lot of times.
They say that there are two Gods, one on Earth (Jesus), second one in Heaven.
Edit: Yeah Redeemer, you may be right, I'm not going to argue.
But could someone tell me, why God needs a son?
In life, we need son to continue our family (genus).
Does God need to continue his genus like this? I'm just curious.

No, God gave his own son (and therefore himself) and sacrificed him for mankind (and therefore himself). This has several implications:

1. No more immolation by the people as in the old testament
2. God gives everything (= himself) for us
3. We are free of sin through this sacrifice
You can think of more things, probably.

I think it is quite logical (not natural, though) that God would come to earth in a mortal body. There is simply no higher sign or gift he could have sent us. Although at that time crucifixion were quite common and I guess people might have suffered more in other circumstances (which I don't want to think about) the idea is unsurpassable: God dying so that we can be near to him again.

All the duality in the scripture might seem weird (stuff like: noone comes to God than through me - Jesus) is consistent if you think of it in the way Reedemer has mentioned. Obviously it makes sense if God and Jesus are the same entity. Also, this means that through him we already have been forgiven all sin and nothing we can do on our end could have achieved that same thing.
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Do Muslims pray to Mohammed?

In Islam we pray only to Allah. We do not pray for prophets, as they are human being as we are.

What he is trying to say is, in Islam the God and the Jesus are two separate entities. Jesus was only human but gifted and blessed by God and was His prophet.

Although in English we refer God as "He", in Islam God and angels does not have a gender.

Also God is not Jesus' father, Jesus was a blessing to the virgin Miriam(Maria). Miriam gave birth to Jesus without any father. This is counted as one of the many miracles of Jesus.

Thank you Quadraxas, I know that my English is sucks

That is the core difference between Islam and Christianity. Christians believe that Jesus is God incarnated, and that he should therefore be worshiped as God. Muslims believe that Jesus was only a prophet who foretold the coming of Muhammad. According to Islam, Jesus is not God at all, but a completely different person entirely.