The central tenant of Christianity is the belief in Jesus as the son of God and the saviour of humanity. But from what does humanity need saving? To answer that, we go back to the beginning.
Christianity teaches that there is only one God and that in the beginning, this God spoke the entire universe into existence. On Earth, God creates two people. A man and a woman named Adam and Eve. They are to tend the garden in which they live and they are free to eat from any tree except the tree of knowledge of good and evil. Sometime later, Eve is tempted by Satan and eats from this tree. Then Adam eats from it too. It is at this point that the two become aware, and understand good and evil; evidenced by a new shame in their nakedness. Disobeying God results in them being expelled from the Garden of Eden, and suffering the effect of death.
Many generations after Adam, God chooses a man named Abraham to make into a great nation. A nation that will one day birth the Messiah that will come to release mankind from the bondage of sin and death. Our modern calendar splits time at the approximate date of the birth of Jesus, who Christians believe is the promised Messiah.
According to the New Testament, Jesus is born to a virgin named Mary, lives a sinless life, and then willingly sacrifices himself as the substitutionary atonement for the sins of mankind. The word atonement is a word used to describe an act that pays for or erases one's sins and transgressions. Three days later, Jesus defeats death by resurrecting form the dead to open heaven to those who believe in him and trust him for the forgiveness of their sins. The resurrected Christ appears to many people over a span of 40 days before he bodily ascends into heaven.
From there he rules and reigns with God the Father and sends the Holy Spirit to teach his followers. It is also maintained that one day Jesus will return to this earth to judge all humans: living and dead, and grant eternal life to those who put faith in him and eternal death for those who don't.
Christians call the message of Jesus Christ: The Gospel, meaning the good news.