What did muckrakers do?

Muckrakers were American journalists in the Progressive Era (the 1890s–1920s) who sought to expose corruption, primarily in politics and business. The name is a reference to Paul Bunyan's Pilgrim's Progress; there is a character within the story referred to as the "The Man with the Muck Rake," who abandoned a chance at salvation in order to turn to filth. Similarly, muckrakers used their investigative skills (rakes) to dig through the polished outer appearance of politicians and big businesses (the muck), thus launching an entire movement of journalism that has yet to slow down.
The most influential people behind the muckraker movement were Lincoln Steffens, Ida M. Tarbell, and Ray Stannard Baker. Lincoln Steffens published what is now considered the first muckraking article, "Tweed Days in Minneapolis," in 1902 regarding the corruption of local politicians. These politicians were working in tandem with big businesses in order to maintain their power and influence the city's treasury.
Ida M. Tarbell's article, "The History of Standard Oil," was published one month after Lincoln Steffen's work. In the article, she revealed unfair business practices behind John Rockefeller's success.
Soon after, Ray Stannard Baker's article, "The Right to Work," exposed the violence that non-striking union members all too often faced when they disagreed with union officials who used their positions in attempts to force agreements.
These three people began the muckraking movement, a movement that is still prevalent in modern journalism. Muckraking is now referred to as "investigative journalism," but its importance in reporting unsavory practices by those in positions of power has not diminished with time.