Christians believe in only one God, but this one God is made up of "three persons" - the father, the son, and the holy spirit, the trinity of Christianity. Christianity is based on the teachings of Jesus Christ (the son) who lived around 2,000 years ago. Christians believe that Jesus Christ was the son of God, his real son. They believe God sent Jesus, his son, to earth, to suffer and die and then be resurrected (brought back to life and raised up to heaven) to both make up for people's sins and to prove to humanity that the teachings of Jesus Christ were in fact the teachings of God, and should be followed.
Through missionaries, the teachings that make up Christianity have spread all over the globe. Today, there are over 2 billion followers worldwide. That's more people than any other religion enjoys.
There are two major divisions or groups that makeup Christianity - Catholics and Protestants. Within that, there are many fragment groups of Protestants, such as Baptists, Methodists, Lutherans, and others. Each of these groups has its own way of looking at things, and its own way of worshipping, but all Catholics and all Protestants believe that Jesus was the son of God and was sent to earth to show us the Christian way of behaving and believing. That's the biggest difference between Judaism and Christianity - Judaism teaches that Jesus was a man, a human. Christianity teaches that Jesus was the son of God.
The Bible, the Christian holy book, is divided into two parts, the Old and New Testaments. Parts of the Old Testament are also sacred to both Judaism and Islam. But all of the New Testament is studied and followed by Christians as it shares the story of the life and teachings of Jesus Christ, the son of God, our Savior.
To learn more about Christianity, see this site: Christianity (Mandy Barrows wonderful site for kids)