The major difference with Christianity, though, is that Christians believe that Jesus of Nazareth, a man who was born around the year 0 (while most scholars now believe the date could have been a couple of years later), was and is divine. The story behind Jesus is that God, the loving, caring, creator of the universe, saw that the world was slowly sinking into it's own evil, and sent his only son to save the people, by offering himself as a sacrifice to cleanse the sins of the people of the earth. This form of sacrifice dates back to early Jewish practices, except that instead of an innocent human, animals were sacrificed to please God and beg for forgiveness.
There is no doubt that Jesus Christ did exist, as three major world religions do make references about him, only in differing ways. The Jews think he was a prophet, not the Messiah (perhaps because his teaching went against many of the things that Jews originally thought), the Muslims think he was a prophet, but that he had little significance besides the fact that he was a wise man, and the Christians think he was divine.
Personal Opinion: For all of you athiests out there, I understand that believing in an all powerful, universe controlling Mega-God may be hard, but look around. Do you honestly think that everything, from the sky, to the earth, to yourself, just happened? It is almost foolish to say that everything just "happened", because there is so much proof against it.
Think of it this way: The Jews are foretold to be God's chosen people. They are, in a sense, protected by God, though it may not seem to always be that way. Want an example? In Jewish history, past and present, Jews have fought in wars after wars, been persecuted, killed, tortured, and nearly wiped out, yet they are still a powerful world religion, mainly in the middle east. Do you see anybody around now who worships Baal, the fertility god? Or Ra, the god of the sun? The answer is no. Those religions died off after a short time, because they had nothing backing them up. In Judaism, Islam, and Christianity, all three worship the same God, just in different ways, with different beliefs.
Now, to the point of Jesus the Christ. Scholars know, without a doubt, that Jesus was an actual man, that he did live, around the year 0.
People think Christians and Jews are so different, when in fact, they really aren't. Christians are just Jews who have realized their Messiah, while Jews think he still is to come.
My own personal belief? God does have a son. His name was Jesus of Nazareth, son of Joseph the Carpenter and Mary. I do believe that he was born of a virgin, and for those who stop there when thinking about it, remember, this is God who we're talking about. If he wants Jesus to be born of a virgin, so be it. If he wants Him to be born from a man... so be it as well, but I really don't think he would do that.
I also believe that Jesus died at the hand of Pontius Pilate, the Roman governor, and after three days, He rose again, as was foretold in Jewish prophecy.
And now, for my final statement to defend my case, think of Christianity in one last way. In the words of C.S. Lewis, let's say that you athiests are right. We die, thats it, nothing more. Everybody loses. But let's say I'm right. You still lose, but I get eternity in Heaven with the God of the universe and his only begotten son.
Let's just say I'm pretty sure of myself.
There are many different denominations of Christianity, and it is incredibly hard to estimate how many Christians there are in the United States because there is no common belief on what defines a Christian. The most liberal view of everyone who says they are a Christian being a Christian would place the figure around 75%, whereas the considerably more conservative belief that anyone outside your own denomination is not a Christian can place the figure as low as 0.1%.
Denominations sprout from the original idea of Christianity because some phrases from the bible contridict each other, and thus to harmonise this people say phrase A is literal whereas phrase B is symbolic. However, one group might say A is symbolic, but another might say B is symbolic. Thus, there are now hundreds of different denominations preaching slightly different versions of effectively the same source material.
Unfortunately, as Christianity is the 'big' religion of the Western World, it gets a lot of negative press. However, like all groups, Christianity has the best and worst among their number. The Christians who get the most press are the ones who give the stereotypical image of never doing anything wrong, and sitting on streets shout 'REPENT OR GO TO HELL', or something similarly pleasant.
However, many Christians - most, in fact - are decent people who just want the right to practice their own religion, like everyone else (or the right to NOT practice a religion, in the case of some).