"Who is Jesus?"
Jesus is the Son of God. Yes, really. God has a Son and His name is Jesus.
"How is Jesus the Son of God? Isn't he the son of Joseph?"
To answer that, let's start with Jesus being born. We'll move on to far more information about Jesus further down below. Jesus is the son of Joseph in the same way that any adopted child is legally the child of the adopting parent. So, yes, Jesus is Joseph's son by adoption. Biologically though, Jesus is the Son of God.
"How did THAT happen?"
God sent His Holy Spirit to Mary when she was still a virgin. You see, Mary and Joseph were engaged to be married, but they were not yet married. God sent an Angel, named Gabrielle, to tell Mary and Joseph that Mary would become pregnant with God's Son. When any woman becomes pregnant, at the very moment of conception, God creates a brand-new spirit to put into that baby. But, when the Holy Spirit of God came down to Mary and touched her womb, God put the spirit of His Son, Jesus, in her womb. That is how Jesus, who was with God long before the universe began, came to be born in Israel about 2,000 years ago.
Jesus was born in a stable. That's right, the Son of God was born in a borrowed stable for animals. You see, 2,000 years ago, the Roman Empire controlled most of the inhabited world. Isreal was under the control of Ceasar. Ceasar Augustus ordered that a census be taken of the entire Empire. Mary and Joseph were ordered, along with everyone else in Israel, to register for the census. When Mary and Joseph came to Bethlehem for the census, all of the rooms for rent were already taken. Poor pregnant Mary gave birth to Jesus, the Son of God and the creator of the universe in a borrowed animal stable. The most powerful being that exists came to Earth as a poor, helpless baby. God's Son came down to Earth in humility to lift us up... to save us, deliver us and heal us. The Creator of all things chose to come down to us as the most humble person who ever lived in the most humble of circumstances. The Maker of us all came down from Heaven to save us from our own sin. To pay for our sin with His very life.