Major differences.
Christian believe in the trinity and say that trinity is part of one God.
Mormons believe in many gods who were once men.
http://www.irr.org/mit/is-mormonism-christian.html
1. Is There More Than One True God?
The Bible teaches and orthodox Christians through the ages have believed that there is only one True and Living God and apart from Him there are no other Gods (Deuteronomy 6:4; Isaiah 43:10,11; 44:6,8; 45:21,22; 46:9; Mark 12:29-34).
By contrast, the Mormon Church teaches that there are many Gods (Book of Abraham 4:3ff), and that we can become gods and goddesses in the celestial kingdom (Doctrine and Covenants 132:19-20; Gospel Principles, p. 245; Achieving a Celestial Marriage, p. 130). It also teaches that those who achieve godhood will have spirit children who will worship and pray to them, just as we worship and pray to God the Father (Gospel Principles, p. 302).
2. Was God Once a Man Like Us?
The Bible teaches and orthodox Christians through the ages have believed that God is Spirit (John 4:24; 1 Timothy 6:15,16), He is not a man (Numbers 23:19; Hosea 11:9; Romans 1:22, 23), and has always (eternally) existed as God â all powerful, all knowing, and everywhere present (Psalm 90:2; 139:7-10; Isaiah 40:28; Luke 1:37).
By contrast, the Mormon Church teaches that God the Father was once a man like us who progressed to become a God and has a body of flesh and bone (Doctrine and Covenants 130:22; "God himself was once as we are now, and is an exalted man, and sits enthroned in yonder heavens!" from Teachings of the Prophet Joseph Smith, pp. 345-347; Gospel Principles, p. 9; Articles of Faith, p. 430; Mormon Doctrine, p. 321). Indeed, the Mormon Church teaches that God himself has a father, and a grandfather, ad infinitum (Teachings of the Prophet Joseph Smith, p. 373; Mormon Doctrine, p. 577).
3. Are Jesus and Satan Spirit Brothers?
The Bible teaches and orthodox Christians through the ages have believed that Jesus is the unique Son of God; he has always existed as God, and is co-eternal and co-equal with the Father (John 1:1, 14; 10:30; 14:9; Colossians 2:9). While never less than God, at the appointed time He laid aside the glory He shared with the Father (John 17:4, 5; Philippians 2:6-11) and was made flesh for our salvation; His incarnation was accomplished through being conceived supernaturally by the Holy Spirit and born of a virgin (Matthew 1:18-23; Luke 1:34-35).
By contrast, the Mormon Church teaches that Jesus Christ is our elder brother who progressed to godhood, having first been procreated as a spirit child by Heavenly Father and a heavenly mother; He was later conceived physically through intercourse between Heavenly Father and the virgin Mary (D&C 93:21; Journal of Discourses, 1:50-51; Gospel Principles, p. 11-13; Achieving a Celestial Marriage, p. 129; Bruce R. McConkie, Mormon Doctrine, pp. 546-547; 742; Ezra Taft Benson, Come unto Christ, p. 4; Robert L. Millet, The Mormon Faith: Understanding Restored Christianity, p. 31). Mormon doctrine affirms that Jesus, all angels, Lucifer, all demons, and all human beings are originally spirit brothers and sisters (Abraham 3:22-27; Moses 4:1-2; Gospel Principles, pp. 17-18; Mormon Doctrine, p. 192).
4. Is God a Trinity?
The Bible teaches and orthodox Christians through the ages have believed that Father, Son, and Holy Spirit or Holy Ghost are not separate Gods or separate beings, but are distinct Persons within the one Triune Godhead. Throughout the New Testament the Son and the Holy Spirit, as well as the Father are separately identified as and act as God (Son: Mark 2:5-12; John 20:28; Philippians 2:10,11; Holy Spirit: Acts 5:3,4; 2 Corinthians 3:17,18; 13:14); yet at the same time the Bible teaches that these three are only one God (see point 1).
By contrast, the Mormon Church teaches that Father, Son, and Holy Ghost are three separate Gods (Teachings of the Prophet Joseph Smith, p. 370; Mormon Doctrine, pp. 576-577), and that the Son and Holy Ghost are the literal offspring of Heavenly Father and a celestial wife (Joseph Fielding McConkie, Encyclopedia of Mormonism, vol. 2, p. 649).