According to mainstream Christianity, who is God? This question also relates to it:
  What is the Biblical basis for the doctrine of the Trinity?


Growing up I've been led to believe that God is God, and Jesus is God's son. But others have also preached that Jesus was God, in a human's body.

And nearly everywhere I go online these days, I end up seeing some website that claims that Jesus is God.

So which is it? Are God and Jesus the same being? Is Jesus God's son? There are some Christian websites that claim different things, so I am confused.

What do the majority of Christians say about this topic?
What you are having difficulty grokking is a concept known as the "Trinity."  The historically correct definition of the Trinity is that God is "three persons in one."

When Jesus says, "I and the Father are One" (in John), he is claiming that he is God.  Historically, God the Father, Jesus (God the Son), and the Holy Spirit are all considered the three persons of the Trinity.  They are all equal, they are distinct, and yet they are all the same.

This link explains the relationship as well as any :)

Don't worry, its a difficult concept, but it is the basis of Nicene Christianity's understanding of how God exists.