Christianity is about God reaching out his hand in friendship to mankind. Christians believe that the whole world and everything in it was created by God. God is a personal being, and by some mystery that we cannot understand, he is three, yet one: Father, Son and Spirit (see the Trinity). He created people in his image, to love him. Yet every person has rebelled against God. Yet instead of turning his back on us, God became a man, Jesus, who was born about 4 BC and lived among us. He taught a message of love throughout Palestine but public opinion turned against him and he was executed as a young man. He died because of us - the death that we should have died. He was crucified on a cross, but he rose from the dead, and now calls us to trust him, to love him and to repent of our rebellion. If we do this, then we our relationship with God becomes restored and we will live forever with God.
Many non-Christians may think that being a Christian is about belonging to a particular church or following a certain set of rule. Most Christians however would agree that at its root, being a Christian means to be in a living relationship with Jesus Christ.