Joseph Smith, the purported translator of the Book of Mormon, declared that it was "the most correct of any book on earth, and the keystone of our religion."

The Book of Mormon presents itself as God-inspired scripture.  It is comprised of a number of sub-books (imitating the organizational style of the Bible), which are said to be written by different prophets throughout the history contained in the book, which spans from approximately 600 BC to 400 AD. An exception to this is the Book of Ether, which is said to date back to the time of the Tower of Babel, and recounts an early migration to the New World by a group known as the Jaredites, who became a vast civilization in the Americas, but were all but decimated by the time the principal narrative of the Book of Mormon begins, in approximately 600 BC.

This narrative is the account of an Israelite migration to the American continent approximately 2600 years ago, and its subsequent development into vast civilizations. 

According to the principal story, a godly Israelite prophet named Lehi (a contemporary of the prophet Jeremiah), was told by the Lord to flee Jerusalem prior to the Babylonian captivity in approximately 600 BC. Lehi, together with his family and a few others, left Jerusalem, traveled through the wilderness to the Red Sea, where they constructed a boat and sailed to the Americas, presumably by way of the Indian Ocean and the Pacific. (Although there is some disagreement, most LDS scholars place Central America as the arrival point and the setting of a large part of the Book of Mormon story.)

Lehi had several sons, among them Nephi and Laman. Nephi was righteous and obedient to the Lord. His older brother Laman, on the other hand, was rebellious and contentious. The two were frequently at odds from the time they fled Jerusalem, and following their arrival in the Americas, the two brothers eventually parted ways, and over the centuries developed into two separate nations, the Nephites and the Lamanites, who were in nearly constant conflict.

The Book of Mormon describes how these two nations grew and filled the land, building great cities and civilizations (particularly the Nephites). The Nephites, it is written, were civilized and generally righteous and godly, and fair-skinned. In contrast, the Lamanites, while more numerous, were primitive, barbaric, and ungodly, and as as a result were cursed by God with dark skin. Throughout the history, the two nations frequently battled and fought with one another.

The Book of Mormon recounts the appearance of Jesus Christ to the inhabitants of the Americas, following his death and resurrection, in approximately 34 AD.  For several centuries following Christ's appearance, the two nations enjoyed peace with one another.  However, war eventually broke out again, and around 400 AD, the Lamanites annihilated the Nephites altogether in an enormous and bloody battle on the Hill Cumorah, which is traditionally ascribed to a hill near modern-day Palmyra, in upstate New York.  However, according to the story, a man named Moroni, who was the son of a Nephite prophet-historian named Mormon, survived the final battle.  Mormon had preserved the ancient history of the Americas, written on golden plates, and Moroni, after the battle was finished and his father had died, hid the plates under a stone on the Hill Cumorah.

In the early 19th Century, a man named Joseph Smith claimed to have been visited by this Moroni, who showed him where the plates were hidden.  Smith then translated these golden plates "by the power and gift of God" into the Book of Mormon, which was first published in 1830.

The LDS Church upholds the Book of Mormon as a literal, factual account of the ancient history of the Americas. It claims that the modern-day descendants of the surviving Lamanites are the Native Americans, who are "a remnant of the house of Israel".

For More Information:

Mormonism in Brief: A bird's-eye view of the LDS Faith

An Interview with LDS President Gordon B. Hinckley

An Open Letter to Mormons: Learn about our motivation!

Quick Reference Guide: Comparing LDS & Biblical Doctrines, Scriptures & Teachings

Testing the Book of Mormon: A Biblical Mandate?