She is an unlikely icon. She hid in back lanes on Bankside, under the South Bank bridges, in the corners of council estates in Islington. She survived just months, sometimes weeks, before disappearing, buffed off or covered over in paint’s endless war on paint. Sometimes she left traces behind, a ghostly shadow of her body or an outstretched arm for those who still sought her out.

Her image spread. From maybe a dozen appearances late in 2003 and through 2004, the little girl and her lost balloon appeared in hundreds of photographs, from blurred cellphone captures to staged shoots. Her copied pixels multiplied around the globe through countless photo-sharing sites, blogs, and web zines. She made a cameo in Woody Allen’s Match Point, filmed in London while she still lived under Blackfriars Bridge. She showed up in tattoos, on running shoes, T-shirts, posters, and wedding cards — copies of copies, made by fans for themselves and, inevitably, for sale. And sell she did. In February 2007, Sotheby’s sold a spray-painted stencil of the balloon girl at auction for £37,200. Her creator, the British street artist known as Banksy, posted a drawing on his website of art buyers bidding on a framed canvas of his scrawled response: “I can’t believe you morons actually buy this shit.”

Modern graffiti emerged in New York in the early 1970s as spray-painted signatures, or tags, still its most common form. Later came throw-ups and pieces — larger, more colourful, more intricate versions of the tag. It is an inherently political art, an assertion of the artist’s existence and, more distantly, of his (occasionally her) right to expression. In graffiti, the art of rebellion takes a back seat to the act of rebellion, to getting up, as often and as brazenly as possible. The graffiti artist has neither the time nor the interest for emotion recollected in tranquility.

Graffiti wasn’t a rejection but an extension of the high art of its century, an art equally interested in shock and action over aesthetics. From Marcel Duchamp’s urinal through Jackson Pollock’s drips to Damien Hirst’s pickled cows, twentieth-century art spurned beautiful imitations for provocative concepts. Some say graffiti is beautiful, that it beautifies the city, just as some say Picasso’s cubist prostitutes are beautiful. But beauty was never the point of either, except as the enemy. Graffiti disfigures the city in the same way Duchamp’s urinal disfigured the art gallery. Mayors understood this; that’s why they fought it. Art dealers understood it, too; that’s why they bought it.

By the early 1980s, graffiti and graffiti-inspired artists like Futura 2000, Jean-Michel Basquiat, and Keith Haring had moved from city streets to gallery walls. Confused by graffiti’s lack of beauty, the commercial world was slower to catch on, but it came around because of the art’s supposed appeal to advertising’s eighteen-to-twenty-five demo. What had once subverted the ubiquity and clarity of billboards became a go-to style for advertisers from ibm to Apple. Today, many first-generation graffiti artists are designers for hip clothing companies or the entertainment industry.

Dressed up by art, pimped out by capital, aggressively hunted in its homeland by New York’s Anti-Graffiti Task Force. In North America and western Europe, traditional graffiti is now a mostly moribund art (elsewhere, for example in Brazil, it’s alive and dripping). But since the early 1990s, graffiti has been reinventing itself with new techniques and aesthetics under the new name of “street art.”

Many street artists began with graffiti, and many continue to use its spray can while adding posters, stickers, and sculpture. They’re still mostly young, for the obvious reasons that the work is usually illegal, potentially dangerous, and doesn’t pay. Men still outnumber women. On average, they’re whiter, more middle class, and better educated than their graffiti predecessors. Perhaps because of that education, graffiti’s political voice has expanded in street art, becoming less about the self and more about the world. It’s typically anti-corporate, though seldom overtly. Street art shares the sheets with culture jammers like Adbusters magazine, but it’s more hopeful than critical.

Following the example set by gallery art, some street art is more about the concept than the art. “Fuck Bush” isn’t an aesthetic; it’s an ethic. Shepard Fairey’s Obey Giant stickers and Akay’s Akayism posters are clever children of Duchamp, ironic conceptual art. But street art generally favours aesthetics, artistic styles that evoke feelings before thoughts. Some is grotesque, some fantastic. Some incites anger, some laughter. Some aims at wonder, the sublime’s territory. The California artist Above, for instance, hangs wooden arrows from utility lines over streets in North America and Europe, pointing up, above the familiar city to the unfamiliar sky.

Oddly — shockingly, in fact — the single most common aesthetic in street art, this child of shock, of defiant tags and disfigured letters, is cuteness. From São Paolo to San Francisco, Tokyo to Toronto, New York to New Orleans, the cute pokes its head through the tangled Duchamps of urban walls. Smiley faces. Pouting faces. Big eyes in big heads. Cartoon characters in cartoon colours. Sad cute. Silly cute. Sexy cute. Helpless. Playful. Innocent. Bambi goes downtown.

Street art’s turn to cuteness is a radical departure not just from traditional graffiti, but from its indoor relative. If beauty had a rough ride in the twentieth century, the cute got kicked to the curb, as impossible for high art to swallow as the disgusting was in Immanuel Kant’s time. “Nothing is so much set against the beautiful as disgust,” wrote Kant in 1764. “Good point,” said twentieth-century art, and went to work tagging toilets, scrawling moustaches on the Mona Lisa, defecating in tin cans, submerging crucifixes in urine, and feeding cow’s heads to maggots.