Without reading the other responses (pressed for time) I will answer your question based on my own understanding and experience (the branch of Christianity with which I am most familiar is the Southern Baptist Convention).
The most important thing about Christianity is, you guessed it, Jesus of Nazareth, called Jesus Christ by Christians. (The word Christ is not part of his name, but is a title meaning messiah, which means a lot of things but for Christians means savior of the world, sent from God. Thus you should know that when you say "Jesus Christ" you are actually stating your belief about who Jesus was)
Anyway, the most important things they believe about Jesus are that he:
1) Rose from the dead three days after his crucifixion and is not currently dead, but in heaven
2) Is fully God, but while he was on earth as Jesus of Nazareth, was also fully man (a case of 100% of both at once)
3) Is one member of the three-member Trinity, which is the confluence of the Father (creator figure), Son (Jesus), and Holy Spirit (less well-defined, but often cited as your conscience).
The last really important belief is that the death of Jesus was a punishment exacted by God on his son (and in a way himself) that would be equivalent to the punishment all humans, ever, deserve for their wrongdoing. The doctrine goes that all people are inherently evil (tend toward doing wrong things) and deserve extreme punishment.
You can think of it as a cosmic imperative, that the evil be punished severely. However, at the same time, it is believed that God loves humankind, and does not wish to punish us, despite our evil (they believe we are his creation, made in his image). Thus, he devised what could be described as a loophole in this cosmic imperative and sent Jesus, his son, also God, to earth to be crucified and rise from the dead.
The act of the crucifixion, Christians believe, was the most painful, severe, punishment anyone has ever experienced. They believe he was not only physically tortured by the cross, but also spiritually tortured by God himself. Basically all of the punishment all people who ever lived deserved added together into one giant, awful punishment.
This is the central message of Christianity - salvation and repentance. They believe that now that the sacrifice has been made, any person may at any time repent of their wrongdoing (sin) and ask forgiveness of God (they also sometimes stipulate that you have to specifically acknowledge Jesus' sacrifice). If you do this, you will not be punished, you have in effect accepted God's gift to you. You will go to heaven if you do this, and you are considered "saved," or, in other words, Christian.
Those who do not are seen as contemptible because they are in essence rejecting a free gift given out of love. Imagine if a stranger died so that your best friend could be spared, and you did not even thank his family. That is how Christians view people who knowingly reject the Gospel (which is the word meaning "good news" which is their message, the message I have just described to you).
One last thing that almost all Christians believe, besides all of the above, is that people have an imperative to do good as best they can, and to spread this gospel to anyone who has not heard it or not yet accepted it. Doing otherwise is seen as irresponsible, resigned friends and family to hell.
So that's it, in a nutshell. There is also an important concept of worshipping God, which is seen as an important thing as well. Worshipping God can be done in numerous ways, by praying and giving thanks, by expressing yourself in music, by doing good works, etc. etc. But if you do not worship God Christians will say that something is wrong with you and that you perhaps did not really accept the gospel, because if you had and really believed it, you would worship God in thanks.
Lastly they believe that everything that exists was created by God including Satan and hell, the earth, heaven, the angels, and all of the physical universe for that matter. They believe that it was create by God in order to glorify himself. They believe that this is not selfish (it would seem selfish at first glance) because God is so good and great that he actually deserves such glory. God is not considered to be anthropomorphic, in that although we are thought to be "made in His image", he is not like us, and is unfathomable to a certain extent. Hence the saying, "God works in mysterious ways."
That's the best summary I've got, any questions?