Christianity is a major religion in the world, and if you don't know a whole lot about the basic principles of the religion, this can leave you in the dark on a lot of things. Many customs, holidays, and beliefs that the United States of America was originally founded on - as well as continue to make us the country we are - are rooted in Christianity.
However, once you learn the basics, Christianity is a relatively easy religion to understand. It's also important to know the differences between the major sub-religions under the umbrella of Christianity in order to understand the faith.
The major thing that separates Christianity from all other religions is the belief that Jesus Christ is our Lord and Savior. Christians believe that Jesus is the Son of God and that he died for our sins. The major religious text for Christians is the Bible.
Christians mostly celebrate the Sabbath day or the day of rest, on Sunday. This is also the day that most practicing Christians attend church. Major holidays celebrated by Christians include Christmas, celebrated as the birthday of Jesus, as well as Easter, celebrated as the day Jesus was resurrected after being crucified, proving him as the Son of God.
These holidays are typically celebrated by attending church services for the occasion and commercially by giving gifts to family members and receiving gives by Santa Claus and Easter Bunny respectively. Gift giving on these holidays is not really a Christian belief, however. This was added later on and stores latched on to that idea for selling merchandise.
Those participating in Catholic mass are usually wearing robes and closely follow all traditions of the mass. Other elements that separate Catholics from other religions under the umbrella of Christianity are the fact that Catholic priests cannot marry, and women cannot become priests.
The Catholic religion does, however, accept nuns who are also not allowed to marry, but who are not afforded the same power as a priest. Because of the incredible tradition that is incorporated into Catholicism, Catholic masses are usually longer than other Christian religions' masses are.
Masses in most Protestant religions are also only held on Sunday. Preachers in Protestant religions are usually allowed to get married and have families, which is another important distinction from Catholicism. Saints rarely appear in Protestant Christian religions, and are not prayed to like they are in the Catholic religion.
There are clearly some important distinctions between Christian religions, but once you understand them, it's simple to understand. Most Christian religions have the same basic principles, but celebrate those principles in very different ways.