Mormonism and Christianity
In broad terms, Christianity is a belief in the atonement of Jesus Christ; that is, that He died for the sins of the world and by so-doing gave everyone the opportunity to be saved, or to return to heaven, or to God's presence, or whatever the case may be. The specifics, of course, vary between each Christian sect, yet they seem to have in common the idea that Jesus Christ is our Savior, and it is by learning and accepting such that one might be excused from his or her mistakes in this life.
Under this generally accepted definition, Mormonism is indeed a form of Christianity. 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.