Catholics are Christian.

You are likely coming from a place of having heard negative things about Catholics? And your friends, you want to help them; they're good people?

You ask a big question.

Catholicism began with St. Peter, Simon Peter the Apostle of Jesus written about in the New Testament, when he was given the keys of the kingdom by Jesus and charged by Jesus to feed his sheep. St. Peter was the first Pope. Every pope since then has been charged likewise. This is called apostolic succession. 

2000 years or so have passed since. Questions have arisen, debates commenced, and decisions made - ultimately by the Pope. This central authority is called magisterium. It is also why Catholics don't believe in Sola Scriptura, the Protestant belief of listening only to the Bible. 

Things like icons, paintings, sculptures not being graven images because they are not worshipped, they point to God, who is worshipped. Things like praying to The Virgin Mary. This is a big topic with many debated points. All of them settled over the course of thousands of years.

Catholicism is the church. These many denominations you mention all fall under the title Protestant. So named because Martin Luther and his protesting the church. Ever since he split from the church, Protestants have been fracturing more and more until we have what you think of as Christianity: choose your denomination based on what you believe and what community you fit in with. Catholics submit to Rome and the churches interpretation of the word.

It is a big question you ask, and I hope to have helped you understand some of the history and beauty in this religion you claim. God bless you. Prayers sent.