Christianity is a very popular religion throughout the world today. It is spreading quickly throughout the world and is a popular religious belief in Korea, so let's just run over a few basics.
Christians believe in one god, called God. Although there is only one god, he exists in three different forms. Christianity believes in the trinity - the father, the son, and the holy ghost.
The Father is God, the creator. He has always been around and always will be. Followers believe he created the Earth, the universe, and everything else.
Secondly, there is the son, Jesus. Jesus is not a separate God, but he isn't human either. God the father created the son, and the son is another representation of himself. It is another form of God. Christians believe God walked on this earth in human form, in the form of Jesus Christ. Jesus Christ was not another god.
Lastly, there is the holy spirit. The holy spirit is another form of God. It is a spirit that exists in all Christians, guiding them through life; telling them what is right and what is wrong. The holy spirit is like a personal guide through life. It is not another god either. It is merely a spirit; more specifically the spirit of God.
All three of these are only one god. The father, the son, and the holy spirit.
Christians believe when someone dies, if they are a saved Christian, then they will go on to heaven - a perfect place where you live forever. Those who are not Christians go to hell, a place of eternal suffering.
In order to go to heaven, a person must be saved. Someone becomes saved by asking God to forgive them of their wrongdoings and to save them and believe in him. If they pray this and mean it, then they become saved. Once a person is saved, they are always saved.
The book of teachings for Christians is The Bible. God told many different men what to write, and they all wrote a book. All of these books were then combined to form The Bible.