Who was William Lloyd Garrison, and what did he write?

William Lloyd Garrison (1805–79) was a leading American abolitionist and, one could say, a standard bearer for a new type of abolitionism. Before the 1830s, abolitionists tended to support freedom for slaves with colonization. The American Colonization Society, founded in 1817, spearheaded the establishment of Liberia in 1822, with the capital of Monrovia, as a colony for freed slaves. The project had substantial political support in the government. Colonization would get rid of slavery but based on the notion of America as a fundamentally white society.
Opposition to colonization found a permanent voice in William Lloyd Garrison and his newspaper, called The Liberator, published in Boston from 1831. Garrison called for the integration of freed slaves into American society as full citizens. His form of abolitionism was militant and uncompromising. He wrote,

I will be as harsh as truth, and as uncompromising as justice. On this subject, I do not wish to think, or speak, or write, with moderation…. I will not equivocate—I will not excuse—I will not retreat a single inch — AND I WILL BE HEARD.

See the first link below for the full text of the article from which this quote was taken.
Garrison promoted the cause through both The Liberator and the American Anti-Slavery Society, which he had co-founded. The abolitionist message was spread through millions of pamphlets, newspapers, petitions, novels, and schoolbooks. The new abolitionists considered slavery to be an unparalleled sin and contradictory to the Declaration of Independence. Both moral/religious and political arguments were made for the abolition of slavery.
The Park Street Church, where Garrison delivered his first public anti-slavery address, is part of the Freedom Trail in Boston. See the second link below.