Mormonism and Christianity
To see how Christianity is related to Mormonism, we must first define Christianity.
1. Broad Definition: Christianity is a religion which preaches the divinity of Jesus Christ, the Son of God. Christians also believe in the Atonement of Jesus Christ; that is, that He died for the sins of the world and by so he gave everyone the opportunity to be saved. Many people, Christian and non-Christian alike, accept this definition.
2. Narrow Definition: Christianity is a religion which accepts the beliefs set forth by the Nicene Creed, a statement of faith written in 325 AD and revised in 381 by a council of bishops. The creed was affirmed in 1974 by a group of religious leaders from around the world and resulted in a document called "The Lausanne Covenant". Together, these creeds acknowledge the doctrine of the Trinity as well as the competency and completeness of the Bible. Fundamentalist Christians generally accept this definition of Christianity.
Catholic and Protestant Denominations
There exist many different Christian denominations as well as a rapidly increasing number of "non-denominational" Christian churches, and more detailed, specific beliefs vary among these denominations. Upon stepping back, however, one can agree that despite many differences, the two groups have in common the belief that Jesus Christ is our Savior and that it is by learning and accepting such that one might be excused from his or her mistakes in this life and be saved in the life to come. In most cases, both Catholics and Protestants accept the Nicene Creed. Both are clearly Christian religions.
Mormonism and the Broad Definition of Christianity
Let us now consider Mormonism: Joseph Smith, the man who organized the religion explained that: "The fundamental principles of our religion are the testimony of the Apostles and the Prophets, concerning Jesus Christ, that He died, was buried, and rose again the third day, and ascended into heaven; and all other things which pertain to our religion are only appendages to it." (Teachings of the Prophet Joseph Smith, p. 121).
Mormons believe that it is through the testimonies of modern-day prophets as well as those of ancient-prophets (contained in the scriptures) that one can learn more about Jesus Christ and thereby gain his or her own assurance that He is truly God's son and the Savior of all mankind. It is, then, by following the example of Jesus Christ and trying to live like Him that Mormons believe they can be forgiven of their sins and live with God after this life.
Another evidence of Mormons' foundational belief in Jesus Christ is the very name of the church. The Mormon church is officially named The Church of Jesus Christ of Latter-day Saints, and it is believed that it is the same exact church as was established by Jesus Christ Himself during His time on the earth. As we read in the Bible, Jesus established a certain organization within His church: He called prophets, twelve apostles, missionaries, and other leaders as were needed in order to teach His doctrine throughout surrounding cities and countries and create a church with a firm foundation. Mormons believe that Jesus Christ has reestablished the same church through Joseph Smith. In the same way that He led Peter, James, John, Paul, and other prophets after His ascension to heaven--that is, through revelation--He continues to lead His church today, through the prophet Thomas S. Monson and the twelve apostles.
Not only, then, do Mormons believe that Jesus Christ is the Son of God and the Savior of us all, but they also believe that His ongoing teachings can be heeded and followed today as He is believed to be the very head of the Mormon church. Mormonism, then, can indeed be called a Christian religion. And just as there are differences between the specific beliefs in each and every Christian sect, the differences in Mormonism in no way affect its being founded on the doctrine of Jesus Christ. To embrace Mormonism, then, is clearly to embrace Christianity, as defined by the broad definition.
Mormonism and the Narrow Definition of Christianity
Mormons do not accept either the Nicene Creed or the Lausanne Covenant. They do not believe in the doctrine of the Trinity, but instead believe in a Godhead of three personages, God the Father, God the Son, and the Holy Ghost, who are one in purpose but separate physical entities John 17:11, 21-23.  They believe the Bible to be the revelations of God to man, and believe the Bible to be the word of God, "as far as it is translated correctly." Mormons believe God has always desired and continues to desire to speak to mankind. Thus, they have more scripture and expect to receive more scripture.
Mormons contend, however, that acceptance of the Nicene Creed should not be the standard for defining a person as a "Christian" as the implications of being "unchristian" extend beyond lack of acceptance of the Nicene Creed. Being "unchristian" implies not believing in the teachings of Jesus Christ and living one's life in a manner that Christ would not approve of.
Mormons are Christians: