When we looked for the words that mattered the most this year, the ones that kept popping up were written on walls: from Syria, where protests erupted after a group of teenagers were jailed for tagging a wall with "The People Want the Regime to End," to Tunisia, Egypt, and Libya, where caustic portraits of tumbling dictators tracked their fall from power. For this In Other Words special graffiti edition, we turned to a prominent expert on the art on its home turf: Roger Gastman, co-author of The History of American Graffiti and co-curator of the recent Los Angeles Museum of Contemporary Art exhibit, "Art in the Streets." Over the next pages, he walks us through some of 2011's most explosive expressions, seen on walls from Tripoli to Cairo to Kabul and beyond.

"For me, how I define graffiti is writing your name over and over again for the sake of fame, which started in Philadelphia in the mid- to late-1960s and in New York City and has continued and spread. When I see those political messages, I call them 'pedestrian' graffiti -- as in, I want to go write on this wall and put a message there, or I am really pissed off about these taxes or this leader doing x, y, and z or not doing x, y, and z. A lot of times these people might be artists, but they are not necessarily graffiti artists. They just see spray paint or smearing oil on a wall or whatever it is as a medium to get their message across. Often, of course, these messages have a much deeper meaning than just someone's name."

Tripoli, Libya, Sept. 1

PATRICK BAZ/AFP/Getty Images

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