“It’s the voice of the city,” says the graffiti artist Paul Insect, “a canvas for people who live or pass through, to speak out.” Insect’s work is part of “Beyond The Streets” at the Saatchi Gallery, an exhibition curated by the graffiti historian Roger Gastman. The show is a rundown of key moments in street art and a survey of artists who led the movement, including the photographer Janette Beckman and the sculptor Felipe Pantone. The most extensive look ever at underground art and culture, “Beyond the Streets” seeks to provoke questions about what constitutes art in the first place. —Bridget Arsenault