What is Uncle Tom's Cabin? How did it help change perspectives?

Uncle Tom's Cabin was a novel written by Harriet Beecher Stowe and published in 1852. Stowe, an abolitionist who actively and fervently opposed slavery, was outraged by the Fugitive Slave Act of 1850. This act imposed heavy penalties on Northerners who harbored slaves or helped them escape. The act imposed requirements on states that they actively pursue and return runaway slaves. Stowe, who like many others, thought slavery would gradually wither and die, was outraged by this law, which helped the cause of slavery and which she considered a step backward in emancipation efforts.
Stowe wrote Uncle Tom's Cabin in response, wanting to show people what an evil institution slavery was. She had lived in Cincinnati, which was right over the border across the Ohio River from Kentucky, a slave state, and was familiar with slavery.
Her stroke of genius was to write a novel of sentiment that played on people's human feelings and to show that even "good" slavery was evil. Part of what the novel does is illustrate that slavery is a cruel institution even in the best of situations. The Shelbys, who own Uncle Tom when the novel opens, are good masters who treat their slaves well, especially the kind and religious Mrs. Shelby. She is the best of owners. Nevertheless, even good owners have the right to do terrible things to their slaves. When Mr. Shelby gets into financial trouble, he sells Uncle Tom, separating his faithful slave from his beloved wife and children. He also has to sell Eliza's four-year-old son, which means ripping the little boy from his mother's arms to who-knows-what cruel faith. In response, Eliza makes a daring and dramatic escape with the little boy. Stowe then shows gradually worsening situations for Tom as his ownership changes hands. Finally, he ends up in the grip of a sadistic monster of an owner, Simon Legree.
Stowe's gripping novel had a huge impact on the slavery issue in the United States. It raised a huge outcry, for many people read it and demanded an immediate end to slavery. It helped abolitionism move from a fringe movement to a central political cause. It is the most striking example in US history of a work of literature having a direct impact on politics. It is often said to have hastened the start of the Civil War by sharply increasing political polarization in the country.
In Europe, as in the US, it was one of the best-selling novels of the nineteenth-century and is said to have spurred the czar to free the Russian serfs. (Initially, however, Uncle Tom's Cabin was banned in Russia.) For more information on Stowe's novel and its impact, you might look at the book by David S. Reynolds called Mightier than the Sword: Uncle Tom's Cabin and the Battle for America. That book also looks at the influence of the novel in Europe.
Stowe's novel is a prime example of the ability of literature, through moving people's hearts, to influence politics and help bring about change.