Christianity is monotheistic religion which worships one patriachal all-powerful God. The Holy Book of Christianity is the Bible, from the greek word Biblios, which translates to 'Book', although technically the Bible is not one book but indeed 66 books.
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).
When someone beats the crap out of you and you become blue/black with bruises. Puffennagen is derived from the italian word 'Puffo' translation of Smurf (little blue gay elf sort of thing that wears white hats and is asexual). Then you add the end part (ennagen) which shows a Germanderivation. When they call you a Puffennagen usually you are a german gay guy or else an asexual.