Short answers, pulled from the story.
The Nullification Crisis was a sectional political confrontation between the state of South Carolina and the federal government in 1832 and 1833, during the presidency of Andrew Jackson. South Carolina declared the federal Tariffs of 1828 and 1832 unconstitutional and void within its borders, and threatened secession if the federal government attempted to enforce collection by force.
South Carolina objected that the protective tariffs burdened Southern agricultural states that imported most manufactured goods while benefiting Northern manufacturers. The state was also suffering severe economic decline following the Panic of 1819, with falling cotton prices and competition from more fertile Gulf Coast lands making recovery slow. John C. Calhoun argued in his 35,000-word "Exposition and Protest" that a protective tariff was constitutionally permissible only to raise revenue, not to shield domestic industries.
Jackson signed the Tariff of 1832 as a partial compromise but refused to accept nullification as a constitutional doctrine. On the 10th of December 1832, he issued a Proclamation to the People of South Carolina calling nullification an impractical absurdity and declaring the power of any state to annul a federal law incompatible with the existence of the Union. He also sent his Force Bill Message to Congress on the 16th of January 1833, seeking authority to use military force to collect tariffs in South Carolina.
The crisis ended when Congress passed both the Compromise Tariff of 1833 and the Force Bill on the 1st of March 1833. The Compromise Tariff, negotiated chiefly by Henry Clay and John C. Calhoun, reduced all rates above 20% by one tenth every two years until reaching a 20% target in 1842. South Carolina's convention repealed the Nullification Ordinance on the 15th of March 1833 but nullified the Force Bill three days later as a symbolic gesture.
Calhoun was the most effective intellectual proponent of state nullification. As Vice President, he secretly authored the "Exposition and Protest" in 1828, a 35,000-word argument that states, acting through democratically elected conventions, could veto unconstitutional federal laws. He resigned the vice presidency to run for the Senate, where he could more openly defend nullification, and later helped negotiate the Compromise Tariff of 1833 that ended the standoff.
The crisis accelerated the formation of the Southern wing of the Whig Party, a coalition united by opposition to Jackson's definition of federal and executive power. It also intensified the connection between states' rights arguments and the defense of slavery, as Calhoun and others increasingly framed the tariff dispute as a proxy for the larger conflict over Southern institutions. Historian Charles Edward Cauthen argued that South Carolina's leaders used the three decades after the crisis to educate the state's population in the principles and necessity of secession, making it the first state to leave the Union in 1860.