"Charm City" is a popular nickname for Baltimore, Maryland (much like New York is "The Big Apple" and Chicago is "The Windy City"). The city's tourist industry and chamber of commerce would like you to believe that Baltimore earned its nickname from visitors who were enthralled with the red brick architecture, bustling harbor, and friendly inhabitants. Indeed, there are many legitimately charming aspects of Baltimore.

However, despite the attempts to put a nostalgic spin on it, the origins of "Charm City" are much more recent and prosaic. The nickname was the brainchild of four advertising executives hired by then mayor William Donald Schaefer. The phrase "Charm City" first appeared in 1975 in a series of advertisements in the Baltimore Evening Sun. The ads were designed to inspire Baltimore residents to visit attractions normally frequented by tourists (this concept of "local tourism" was something of a pet project for Schaefer, and one that he would enact on a larger scale when he became the governor of Maryland).

Baltimore has older, slightly less beloved nicknames that it has earned in a more natural manner, rather than through the modern miracle of marketing. During the War of 1812, the city became known as "Mobtown" due to violent rioting caused by tension between the Federalists and anti-Federalists. During the same time period, the British dubbed Baltimore "Nest of Pirates," referring to the thriving practice of privateering that sustained the city's economy throughout the war.

Of course, all of these nicknames are unofficial. At the moment, the official nickname/slogan of Baltimore is the wholly uninteresting "The Greatest City in America" (a slight improvement over the previous official nickname "The City That Reads," which was changed after people began calling it "The City That Breeds" due to Baltimore's unusually high rate of teen pregnancy and "The City That Bleeds" because of the escalating murder rate).

Sources:
http://www.washingtoncitypaper.com/archives/cover/1999/print_cover0910.html
http://www.mdhs.org/library/Image15.html
http://www.ci.baltimore.md.us
http://www.bcpl.net/~etowner/battle.html