Frederick Douglass was the most famous and perhaps the most influential abolitionist of his time. He was born into slavery, and wound up as a family slave in Baltimore. In this setting he learned to read and write, and eventually, after enduring a particularly brutal stint as a plantation slave, he escaped from slavery in Baltimore. He made his way to Boston, where he befriended William Lloyd Garrison.
In Boston, he wrote an autobiography, partly to prove that he had indeed been a slave despite his formidable eloquence, and, as a leader in the Massachusetts Anti-Slavery Society, toured the nation and England lecturing like-minded Northerners. His abolitionist newspaper, the North Star, rivaled Garrison's Liberator in the anti-slavery community.
When the Civil War broke out, Douglass urged President Lincoln to issue an emancipation proclamation, and to allow former slaves to fight in the Union Army. As the war came to an end, he pushed for the Fourteenth and Fifteenth amendments to protect the civil rights of freedmen. As Reconstruction spiraled into a nightmare for blacks in the South, Douglass continued to advocate for their rights. He was also involved in several other causes in his time, particularly womens' suffrage. In short, Douglass is famous because he was perhaps the leading advocate for social justice during his lifetime.