Doctrine and Covenants, to quote its explanatory introduction,
is a collection of divine revelations and inspired declarations given for the establishment and regulation of the kingdom of God on earth in the last days.
Although most of the sections are directed to members of the Church of Jesus Christ of Latter-day Saints, the messages, warnings, and exhortations are for the benefit of all mankind, and contain an invitation to all people everywhere to hear the voice of the Lord. . . speaking to them for their temporal well-being and their everlasting salvation. The book of Doctrine and Covenants is one of the standard works of the Church in company with the Holy Bible, the Book of Mormon, and the Pearl of Great Price.
That's what the Mormon Church claims it is. In reality, Doctrine and Covenants is a compilation of messages from Joseph Smith designed to bolster his image as a prophet. Although he claimed to be receiving direct revelations from God, these "revelations" often contradicted others given in the Book of Mormon and elsewhere. They were simply a convenient way for Smith to get the things he wanted (such as many wives--see Doctrine and Covenants section 132:1-62) without argument or interference. After all, who would want to argue with God?