This has been in the news a lot. I thought I'd give my take on the question since Mormon is the nickname people use for the church I belong to. The official name of the church is The Church of Jesus Christ of Latter-Day Saints. Why do people call us Mormons? We believe in the Bible as the word of God: God spoke to ancient prophets and these are their writings. The Old Testament was written by ancient prophets before Jesus Christ's time. Most of those writings pointed toward the coming of a Messiah, Jesus Christ. The New Testament, the other part of the Bible, are writings written by followers and apostles of Jesus. Some actually knew him, others were 'converts' who became apostles or followers of Jesus after his death (ie., Saul-Paul). We also use a book of scripture called the Book of Mormon, another Testament of Jesus Christ. Where did it come from?
During the reign of King Zedekiah (around Isaiah's time--Old Testament) a man named Lehi and his family left Jerusalem, before it was going to be destroyed, bringing ancient writings with them. They also wrote of their journeys and travel to the Americas. These writings were on plates made of a brass material. The leaders passed these sacred records from generation to generation. Within the plates the story is told of Jesus Christ visiting the Americas after his death and resurrection, healing and teaching the people as he did in Palestine. Eventually the plates were buried by the last prophet in the Americas. Later these plates were given to an unlearned man named Joseph Smith which he translated by the power of God.
If you have read the Book of Mormon you know it completely supports the teachings of those Biblical prophets. It testifies that Jesus is the Christ, the Messiah and the Son of God. Now how can you know it's true? Do we have the plates in a museum? No. We don't. But we do know God is our Father and he wants everyone to know the truth for themselves. How can we know the truth for ourselves? By praying. Study the scriptures for yourself--the Bible, The Book of Mormon--and pray about their truthfulness. God knew people would doubt the Bible, so he prepared an additional way for us to know about his son Jesus Christ and his miraculous atonement.
Yes, 'Mormons' are Christian. Yes, we have a different beginning to our church. It was not invented nor broken off from another church. If you want to know more go to mormon.org -- it's a way for you to learn more. I am sharing just a small part of the story. But it is my testimony that I know that Jesus Christ lives and loves each one of us. He is there to help us. We can come to know Him and our Father through the scriptures and prayer. They will send the spirit, sometimes called the Holy Ghost or Holy Spirit and it will testify, or tell you, that it is true. That is the beauty of it, you can know for yourself and you don't have to rely on anyone else. If you don't want to know that is your choice and I respect that but please don't misjudge 'Mormons' as a cult or not Christian without actually learning anything about us.
In high school I had a friend who told me I wasn't Christian when he found out what church I belonged to. I assured him I was Christian but he would not listen me. I found this very frustrating but I understand that it could be very threatening to other churches to have a church that tells it's people they can know the truth for themselves. We worship Jesus Christ--he is the center of our doctrine. Those who say otherwise have never studied it. So again I ask you to find out for yourself, thanks.