That is a fairly epic event, so let me sum up. The Mormons were driven from their settlements in Nauvoo, Illinois, and Liberty, Missouri, and other places, due to persecution including an extermination order from Governor Lilburn Boggs of Missouri which allowed for the killing of all Mormons that were not out of the state by a certain date. As a bit of trivia, the extermination order remained on Missouri state lawbooks until about 1978 or so, although it was obviously not enforced. I served a mission for the church in Indepencence, Missouri and I never experienced any hostility. Missourians are good people. The journey itself involved a lot of pain and death, as it is difficult to walk hundreds of miles over the rocky mountains without the benefit of paved roads and only one pair of shoes, using wagons and handcarts (a kind of small, human-powered wagon). Pioneers set out in different groups at different times and each group encountered their own hazards. Many of the pioneers succumbed to exposure and exhaustion. Brigham Young led a group of pioneers and upon arrival at what is now the Salt Lake valley, said, "This is the right place". I hope that helps.

During it's first two decades, the Mormon church had numerous conflicts with it's neighbors. The church left New York, then Kirtland, Ohio, then MIssouri, and finally Nauvoo, Ill before it found a peaceful home in Utah. You can read more about some of these conflicts and the reasons for them here: http://www.lds-mormon.com/tmpc.shtml The church found peace in Nauvoo, Ill for nearly five years, until it was discovered that church founder, Joseph Smith had secretly married some 30+ women and girls. Besides being illegal, many found it especially offensive that these girls were as young as 14 years old and many of the women Smith married, already had husbands. http://www.wivesofjosephsmith.org In anycase, some members of Smith's church tried to get Joseph to obey the laws by abandoning polygamy. Smith refused. These members then decided to publish what they had learned in a newspaper called the Nauvoo Expositor. When the paper hit the streets, Joseph Smith was not happy. He ordered the printing press to be destroyed. Freedom of the press was a serious issue in early America. Joseph was arrested and jailed. Before he could stand trial a mob from neighboring towns angry about the way Joseph had behaved, stormed the jail and shot him. http://www.wivesofjosephsmith.org/2425-SarahandMariaLawrence.htm http://www.lds-mormon.com/06.shtml Mormon leaders decided that in order to have peace and practice polygamy as they wished, they would need to isolate themselves away from other people. The church left Nauvoo, and traveled some 1000+ miles to Utah.