Nauvoo is most famous for having been founded in 1840 by Joseph Smith, Jr., as one of the first centers of The Church of Jesus Christ Latter-day Saints, following their exodus from Missouri in the wake of the Mormon War. In 1844, owing to hostility between the Mormons and their neighbors, as well as worries over a perceived theocracy in Nauvoo, the Illinois Legislature stripped the town of its legal status, leaving Nauvoo as a large (~12,000 residents) unincorporated community de facto ruled by the LDS Church. By 1846 hostilities (including vigilante violence and the assassination of Joseph Smith, Jr.) had grown severe enough that Brigham Young (accepted by most of Smith's followers as his successor) and his supporters left in what is known as the Mormon Exodus, along the Mormon Trail.
The temple, which was the second the Mormons built after the original (which in turn was destroyed in the Mormon War) in Missouri, was burnt down by arsonists shortly following the Mormon Exodus. But in 1999 the temple was built and completed in 2002. The exterior is a faithful reconstruction, but the interior (which is off-limits to non-Mormons) follows the architectural plan of a modern LDS temple.
Nauvoo is on Illinois state road 96, part of the Great River Road following the Mississippi River. Driving to Nauvoo from St. Louis or Chicago takes about 3 hours, depending on traffic. St. Louis and Chicago are the nearest cities with full air service; Peoria and the Quad Cities are closer and are reached by some but not all commercial airlines. Quincy is closer still, and has been served by commuter airlines at various times, but the service is intermittent.
Drive, bike or walk; the town is too small to pose difficult transportation problems.