I asked the same myself almost 9 years ago when I was curious what my friends (who were Mormon) believed. I have since become a member of the Church of Jesus Christ of Latter-Day Saints. I have over the last 8.5 years studied the scriptures (Bible and Book of Mormon) about 2-3 hours a day. -I don't claim to know everything, but I am knowledgeable in this subject. Not to slight others who have responded to your question, but I don't think many of them can say the same.
Now, to answer your question. The Book of Mormon contains numerous discourses from various ancient prophets that were descendants of some of the "Lost tribes of Israel" spoken of in the Old Testament. These tribes were scattered across the earth, some of which were led to the ancient Americas (which is the setting of the Book of Mormon). It also contains fragments of the religious and civil history of these peoples.
Page for page, the Book of Mormon references Jesus Christ more than the Bible (a fact which those who have not read the Book of Mormon completely, or the Bible for that matter) are ignorant of when they accuse members of the Church as being "not Christian."
Key teachings include:
*salvation comes only through the name of Jesus Christ.
-Mosiah 3:17, 2 Nephi 25:20. Compare to John 14:6 and
Acts 4:12 in the Bible
*We are all children of God our Heavenly Father and he loves and knows each of us individually.
-1 Nephi 11:17. Compare to Acts 17:29 in the Bible
*Through faith in Christ and repentance, we can not only be
forgiven of our sins and weaknesses, but overcome them
-3 Nephi 27:20, Alma 26:12. Compare to Hebrews 11:6 and
Philippians 4:13 in the Bible
I can list many more topics and many more references, but the Book of Mormon's subtitle does the best job of summarizing its content: "Another Testament of Jesus Christ." I am currently reading the Book of Mormon through for the 23rd time and have read the Bible numerous times as well. I find rich insights in both as I combine what I have learned.
There will always be nay-sayers to listen to, but honest, intelligent, and truth-seeking people usually go to the source. You don't go to a Chevy dealership to learn about Fords. It is ironic that many of those who profess to be Christians are so quick to cast stones at a book that supports the canon (scriptures) they base their faith in.
If you are still curious about the Church and want official information, visit www.mormon.org.
Monday, December 3, 2007