The phrase is attributed to Edmund Burke (1729 - 1797), a British politician, as quoted in Thomas Carlyle's book, "Heros and Hero Worship in History" (1841):
Burke said that there were three Estates in Parliament, but in the Reporters Gallery yonder, there sat a fourth Estate more important far than they all.In Britain, the three branches (estates) of government referenced Parliament: The House of Lords (the Lords Temporal and the Lords Spiritual - nobles and clergy) and the House of Commons.
In modern times, the "press" has been expanded to include all news media, not just newspapers.
Why Should You Care?
The First Amendment to the Constitution "frees" the press but carries with it a responsibility to be the people's watchdog.
However, the traditional newspaper (if there really is such an animal) is threatened by shrinking readership. Television is focused on entertainment, even when it dresses it up as "news." Radio is threatened by satellites. All are confronted with the frictionless distribution enabled by the Internet, the disruptive effects of digital information. None have figured out a business model that pays for content at today's rates.
Bloggers may be great at filtering and framing information, but few have the time or resources to perform acts of investigative journalism.