God is tripartite: God the Father, God the Son, and God the Holy Spirit. The penultimate event in human history was when God the Son assumed human identity in the form of Jesus of Nazareth. He lived a perfect life and then allowed himself to be crucified by the Jews and Romans then in power in Judea. Ultimately, the crucifixion was a crucial part of the original plan to redeem humans from the sin they fell into in the Garden of Eden after God had created perfect humans in a perfect world. Each human falls into sin himself/herself through their own choices recapitulating Adam and Eve's original sin. Jesus assumed humans' sins and paid the retributive price to God the Father for them. Jesus was then resurrected from the dead three days later to overcome death for all humans and to verify his divine identity to humans.
Jesus loves all people, including those who have ridiculed him on this website. He reaches out in love to them. All it takes for a relationship with him is to accept his sacrificial death on the Cross to cover our sins from God the Father. This acceptance is a pure act of faith that does not first require attempts to justify oneself in God's eyes. God simply requires that a person accept his Son as his/her Savior and his/her Lord.
If a person rejects God by rejecting his Son, then God will grant the wish of the person to be left alone for eternity. But when God's presence is not manifest, then nothing good is present to be enjoyed. People in Hell are quarantined from the rest of God's Creation which enjoys his full presence. These people receive only just punishment for their sins committed while on Earth.
The act of faith that reconciles the person with God is a rational act that is supported by the historical accuracy of the Bible and the verification of such miracles as Jesus' resurrection (if he did not rise from the dead, then why did not his adversaries display his dead body? If the disciples stole his body, why did they willingly allow themselves to be tortured to death when they could have revealed what actually happened to his body and cut a deal with the Roman and Jewish authorities?) As Pascal observed, belief is rational because it is a good bet--nothing to lose and everything to gain, while the non-believers' payoff matrix is the reverse of the believers' matrix. As William James observed, this decision is required by each of us whatever one might decide.
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).