The Big Question: Just who is Banksy, and what is all the fuss about his work?

Why are we asking this now?

Although his guerrilla street art has made him one of the most feted artists of our time, Banksy's identity has remained something of a mystery. His works appear on walls in London, Bristol and beyond as if by magic. Now, for the first time, a photograph snatched by a passer-by appears to show Banksy at work.

The grainy mobile phone image taken near the Bethnal Green Working Men's Club in east London shows a man dressed in jeans and trainers, painting double yellow lines which trail off the road, across the pavement and up a wall.

A spokeswoman for the artist refused to confirm the photo was of Banksy, but said the painting in the spot, showing a man in overalls resting after painting double yellow lines that turn into a flower, was definitely his work.

The painting is thought to be a response to Tower Hamlets Council, which has pledged to remove his graffiti from the borough.

Why has it taken this long to photograph him?

Operating on the fringes of the law, Banksy has deliberately avoided being photographed in public and has kept his biographical details vague.

It is believed that he was born in 1974 in Bristol, home to many of his early works, and that his real name is Robert or Robin Banks. There have been several purported photographs of Banksy, but despite his increasing fame, the media have not yet managed to pin down his identity. In April, a Banksy "self-portrait" featuring a stencil of a chimp's head sold at auction for £198,000. Even Banksy's parents are said to believe he earns a living as a painter and decorator.

Is it just graffiti?

Tower Hamlet Council certainly thinks so – it has promised to rid the borough of Banksy's graffiti, saying: "Whilst some graffiti is considered to be art, we know that many of our residents think graffiti in areas where they live, such as local housing estates, is an eyesore."

Neighbouring Hackney Council has also said it will remove his work, insisting: "We can't make a decision as to whether something is art or graffiti. The Government judges us on the number of clean walls we have."

Over-zealous street cleaners have also mistaken his work for graffiti. In March, against the explicit orders of Bristol City Council, Nordic, the company contracted to work on graffiti removal, painted over a 25ft Banksy mural on a row of garages in Albion Road, Easton, in Bristol, which had been there for 10 years, in thick black paint.

But on the other hand, Banksy is widely loved by the public, and only a handful of street artists, led by Jean-Michel Basquiat – the New-York artist who died of a drug overdose in 1988 – have commanded such high prices for their work.

Is he a worthwhile artist?

Much of his work has a political message – anti-war, anti-capitalist and anti-establishment. He has painted a hole revealing blue sky – one of nine stencils – on the Palestinian side of Israel's separation barrier in the West Bank and deposited an inflatable doll dressed as a Guantanamo detainee at Disneyland.

But it is Banksy's ingenuity which has made him the darling of collectors, including Damien Hirst, who sealed the graffiti artist's reputation when he included his pieces in a 2006 exhibition of his personal art collection at the Serpentine Gallery in London, In The Darkest Hour There May Be Light. His Barely Legal exhibition in Los Angeles in September 2006, including an Indian elephant painted in a pink floral wallpaper pattern, brought him international attention. Brad Pitt and Angelina Jolie are fans, as is Christina Aguilera, who bought three prints including Queen Victoria sitting on a woman's face in 2006.

Last week, a sale of 10 of his works at the auctioneers Bonhams fetched £500,000 – £200,000 more than the estimated price. The most expensive piece in the auction was Avon and Somerset Constabulary, an image of two policemen looking through binoculars, which was estimated at £60,000 to £80,000, but sold for £96,000. The pieces that sell at auction are mainly painted on canvas, board and metal. His wall art is more difficult to place a value on, although earlier this year, thieves used a chisel to remove a stencilled silhouette of a rat bouncing a beach ball from a wall in Paddington, west London, which was put up for sale on eBay for £20,000. The auction site was forced to take it down.

What is the appeal?

Lazarides Gallery in Soho, the main dealer of Banksy's work, describes him as "a media star for stunts like hanging his own work in the Tate gallery, but a popular star long before any of this high-profile activity – simply because the people love his stuff". According to Lazarides: "Banksy has not only brought UK graphic art to an international audience but has proven, through his emotional and provocative work, that his generation are not the apathetic and unfeeling demographic they are made out to be."

Louisa Buck, the contemporary art correspondent for The Art Newspaper, said: "Everybody loves a maverick. The fact that Banksy was taken up so whole-heartedly by Damien Hirst has upped his market currency immeasurably." But she added: "Among the art world the consensus is his work looks better out on the street than on gallery walls, but that's not going to stop people spending enormous amounts of money."

Gareth Williams, the senior picture specialist at Bonhams, which has sold many Banksy works, said: "The appeal is down to the whole phenomenon that surrounds him – a relatively unknown graffiti artist from Bristol who has taken the art world by storm, but has done it on his own terms. Also because his images are so accessible, iconic, democratic and humorous, his work reaches people on a simple level."

How can you tell it is a Banksy?

His works have been found across the capital as well as in his home town of Bristol and in Brighton. Most of his graffiti art is in his trademark stencilled style and he repeatedly returns to favourite subjects, such as rats and monkeys.

As well as subversive political messages, his work is often strongly humorous, for example the stencil he painted on the steps of Tate Britain warning "Mind the crap". It took the British Museum a whole eight days to notice when he deposited a chunk of "rock art" in one of its galleries, depicting a stone-age hunter with a shopping trolley, with the caption "Banksyus Maximus".

In another of a series of works smuggled into major art institutions, he hung a parody of Andy Warhol in New York's MoMA, in which the iconic Campbell's soup can had been replaced with cans of Tesco Value soup.

In the US, he painted the legend "Fat Lane" on the sidewalk at Venice Beach, in a poke at America's obesity problem. For this year's Glastonbury Festival, he produced Boghenge, a replica of the Stonehenge circle made out of mobile public lavatories.

Is Banksy an important artist?

Yes...

* The art market is willing to pay handsomely for his work. His most expensive piece, Space Girl and Bird, fetched £288,000 in April

* His work brings humour and escapism to everyday life, as Bristol City Council has recognised by protecting several of his pieces

* In a recent poll of 18 to 25-year-olds, Banksy was ranked young adults' third favourite "art hero" after Walt Disney and Peter Kay

No...

* Defacing private property is a criminal offence and Banksy could be encouraging other, less talented imitators to commit a crime

* Much of his art is disposable. In April, cleaners painted over a mural of John Travolta and Samuel L Jackson toting bananas in London

* The veteran critic Brian Sewell said: "He is a complete clown and what he does has absolutely nothing to do with art"