Christianity revolves around the belief of the teachings of Jesus Christ. Christianity also has different branches, including catholicism (popularly-believed to be a radical and strict form of christianity), lutheran (said to be christianity for blacks), protestant, and presbyterian.
Unfortunately, christianity has been perverted by the world to be an awful religion based around occult-relations....this is because there are stupid people who claim to do bad things in God's name (see Al Qaeda & KKK; the KKK discriminates in the name of christianity).
Christianity revolves around love for everything, including fellow human beings. Unfortunately, we as human beings are doomed to hate one another because of our nature. Nevertheless, there are those who try to follow christianity strictly, so much that they don't say words like "damn" or "fuck" (christianity has been popularized as a personality where people don't swear).
On a sidenote, the dominant religion now is a battle between christianity and islam. Many are converting to islam for countless reasons; it is apparently a fascinating religion, albeit strict (and you're forbidden to eat pork).
"Do unto others as they would love to have done unto you."
"Love your enemies....for if you love only those who love you, where is the reward in that?"
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).