Joseph Smith received a revelation from an angel. He was told where to find some Golden tablets. The tablets required special glasses which were provided to read. He then lost the glasses and was delivered seer stones to read the tablets. He then had a friend transcribe the text of what became the book of mormon, while he translated the tablet.
The Book of mormon is ever changing. The first few drafts were so rife with mistakes that they Church changed the book to make it easier and more cohesively assembled.
The crux of the religion is it is works based. Mormons do plenty of good works, and the hope is they can acheive Godhood like God did, because he started as a man also.
They also believe Jesus and Satan were heavenly brothers and the rift in Heaven was over who was going to be designated as Savior of the Human Race.
Men used to be able to have more thna one wive, now they do not, but the man still has the authority to bring his wife into Heaven or not, once he acheives Godhood. He is then allowed to serve as God over a Planet.
They believe that the God had to physically lay with Mary.
They also believe some funny things about how the planet is to be populated once you achieve Godhood. I have relatives that are Mormon and they are the nicest people in the world.
Not saved, but very nice.