The 'Muckrakers' style of journalism articulated reform, reform, reform. Beginning in the late 19th century and gaining popularity through the first decade of the 20th century, their agenda was to raise public awareness. It is important to note that this style of journalism was met with rising literacy rates, which meant public opinion would eventually get louder. This style of journalism inspired Americans to take action against the ills of society created by the industrial revolution. For example, the stratifications between big business and those they employed, child labor, relationships between big business and political machines, unhealthy and unsanitary working conditions to name but a few. The influence of well known muckrakers such as Upton Sinclair, Ida Tarbell and Jacob Riis were instrumental in the passage of important congressional legislation. Two of the best well known laws are the Food and Drug Act and The Meat Inspection Act. These laws along with many others were passed due to the publics' outcry for reform. The public was educated about these social ills due to this new brand of journalism. By today's standards this type of journalism is regarded as the 'investigative reporter'.
I wanted to add a specific detail. Roosevelt was VERY impressed with Lewis' "The Jungle," so much so that it is often directly linked to the Pure Food and Drug Act of 1906.
It is easy to find information about this, but you might want to start with a letter from Roosevelt to Lewis: http://www.teachingamericanhistory.org/library/index.asp?document=1299
Muckraking journalism changed the U.S. in profound ways. The best example is found in Sinclair's "The Jungle" which prompted immediate federal intervention into the meat-packing industry, and the food industry in general. It was a time of great opportunity and with that came opportunities to make profits illegally. This heightened level of dishonesty and deception in the corporate world led to the muckraking movement. Since television had yet to be invented and movies were in their infancy, journalism was the only way most people could get their news.