According to the Book of Mormon, both they and the Lamanites descended from Lehi, a Jew who came to America in 600 B.C. The Nephites were his faithful descendants, who became corrupt, and were finally destroyed by the Lamanites. Much of the Book of Mormon is a description of the wars between these two groups.
Dictionary of "Mormonese"
Stop talking past each other. Gain a better understanding of the words that are unique to Mormonism and the differences of shared terms between Mormonism and Christianity.