Short answers, pulled from the story.
The Missouri Compromise admitted Missouri as a slave state and Maine as a free state, and banned slavery in the remaining Louisiana Purchase lands north of the 36 degrees 30 minutes north parallel. The 16th United States Congress passed it on the 3rd of March, 1820, and President James Monroe signed it on the 6th of March, 1820.
Senator Jesse B. Thomas of Illinois proposed the key geographic restriction, and Speaker of the House Henry Clay of Kentucky built the majority needed for passage. President James Monroe signed the legislation on the 6th of March, 1820.
Admitting Missouri as a slave state without a matching free state would have given slave states more than eleven senators, breaking the sectional parity that had been maintained through paired admissions since 1815. Linking Maine's admission as a free state to Missouri's entry preserved the balance at eleven states each.
Representative James Tallmadge Jr. of New York proposed in February 1819 to prohibit further introduction of enslaved people into Missouri and to free all children born to enslaved parents there at age 25. The House passed both provisions along sectional lines, but the Senate rejected both, with the slave-migration restriction losing 22-16 and the gradual emancipation provision losing 31-7.
In an April 22 letter to John Holmes, Jefferson called the compromise line a fire bell in the night and described it as the knell of the Union. He warned that a geographical line coinciding with a marked moral and political principle, once drawn and held up to the angry passions of men, will never be obliterated.
Stephen Douglas's Kansas-Nebraska Act of 1854 effectively repealed the Missouri Compromise by removing the ban on slavery north of 36 degrees 30 minutes. The Supreme Court then declared the Compromise unconstitutional in Dred Scott v. Sandford in 1857.