Oh American christianity is something different as a whole.

I'll give my best summary in as few words as I can during my lunch.

Christianity is an Abrahamic faith with its roots in catholicism. When Martin Luther, a catholic from Germany, wrote a large 95 point thesis detailing his problems with catholicism and how the church had been warped from its intention, this lead to whay most modern Americans would call Lutheranism.

This reformation of the church that started with Martin Luther is known as the Protestant Reformation, protestants being anyone who believed in Christ but not in the orthodox or catholic belief set and rites.

The separation of that faith and the pursuit to practice it openly (sometimes even if it was MORE restrictive than the existing systems) led to the exodus of religious groups to America. This is where some Americans get the idea that "america was founded on Christianity and religious freedom", as these were protestants who were escaping religious persecution for rejecting mainline catholicism.

Some time down the line, I don't know the history of this part, the general term for anyone believing in christ but not catholicism (some going as far as saying lutheranism is catholic-lite as they still practiced communion and most protestants don't recognize communion or any of the 'rites' as those are things they see as placed on top of religion by man and not by God) was just left at Christians.

For all intents and purposes, in the US, Christian mostly means Protestant, methodist, Lutheran, Baptist, Southern Baptist (they're separate, I was raised Southern Baptist, they get pissy about being lumped in), calvinist, and non-denominational (people who don't claim a 'branch' but worship christ in a way not easily or intentionally not tied to a singular rhetoric.

Please ask more questions and I'll try my best. Having been born to a Southern Baptist family, it's a topic I love to discuss and learn more on. I'm not religious though, so hopefully none of my stuff comes off disrespectful, just happens to be a family trade I can't quite put down.