A journalist was someone who specialized in journalism: writing articles for a newspaper, magazine, or other periodical. Journalists who specialized in researching and disseminating information about events were called reporters.
In 2371, Captain Kathryn Janeway and Tom Paris encountered Latika, a young boy, whose father was a journalist. Latika proclaimed that he, too, was a journalist, writing for his school newspaper. (VOY: "Time and Again")
In 2372, Neelix became interested in journalism and tried to become a journalist. He even created his own program, A Briefing with Neelix, which was broadcast throughout the USS Voyager. He later investigated the rumors of a spy aboard Voyager, which turned out to be Michael Jonas, whom he later fought in engineering. (VOY: "Investigations")
Jake Sisko was a journalist for the Federation News Service and reported on the Dominion occupation of Deep Space 9 and the First Battle of Chin'toka. (DS9: "Call to Arms", "A Time to Stand", "Rocks and Shoals", "Tears of the Prophets")