Satan, also called the adversary or the devil, is the enemy of all righteousness and of those who seek to follow God. He is a spirit son of God who was once an angel "in authority in the presence of God" (D&C 76:25; see also Isaiah 14:12; D&C 76:26–27). But in the premortal Council in Heaven, Lucifer, as Satan was then called, rebelled against God. Since that time, he has sought to destroy the children of God on the earth and to make them miserable.
One primary issue in the conflict between God and Satan is agency. Agency is a precious gift from God; it is essential to His plan for His children. In Satan's rebellion against God, Satan "sought to destroy the agency of man" (Moses 4:3). He said: "I will redeem all mankind, that one soul shall not be lost, and surely I will do it; wherefore give me thine honor" (Moses 4:1).
Satan persuaded "a third part of the hosts of heaven" to turn away from the Father (D&C 29:36). As a result of this rebellion, Satan and his followers were cut off from God's presence and denied the blessing of receiving a physical body (see Revelation 12:9).
Heavenly Father allows Satan and Satan's followers to tempt us as part of our experience in mortality (see 2 Nephi 2:11-14; D&C 29:39). Because Satan "seeketh that all men might be miserable like unto himself" (2 Nephi 2:27), he and his followers try to lead us away from righteousness. He directs his most strenuous opposition at the most important aspects of Heavenly Father's plan of happiness. For example, he seeks to discredit the Savior and the priesthood, to cast doubt on the power of the Atonement, to counterfeit revelation, to distract us from the truth, and to contradict individual accountability. He attempts to undermine the family by confusing gender, promoting sexual relations outside of marriage, ridiculing marriage, and discouraging childbearing by married adults who would otherwise raise children in righteousness.
Individuals do not have to give in to Satan's temptations. Each person has the power to choose good over evil, and the Lord has promised to help all who seek Him through sincere prayer and faithfulness.