THE scene outside Kate Middleton's flat in Chelsea last Tuesday was a predictable one. At 6am, dozens of tenacious journalists - TV crews and photographers - jostled with one another for position. What they wanted was a picture or clip of England's Prince William's girlfriend on her 25th birthday, leaving for work.

And hours later, those unspectacular pictures were splashed across newspapers and websites all over the world.

Unpleasant, you might think, for an amiable girl such as Middleton.

Prince William certainly does. He begged for the paparazzi to "stop harassing her."

Stop the paparazzi? Impossible, surely. Stop who, exactly? What are "paparazzi images" for a start? And who are these nefarious photographers lumped together under that seamy title of "the paparazzi"?

The trouble is, not everyone understands. Is it the destination that defines the picture's status?

Does it make any difference if I am looking at an unsolicited photograph of Drew Barrymore in Heat, or in Vanity Fair? Or is it the origin of the photograph that counts? Is it a paparazzi shot only if taken by a freelancer? Or is it about the chase?

For every photographer who takes a punch from Chris Martin, there's one who's making a tidy sum with a pre-arranged "chance encounter" between star and snapper.

The term 'paparazzo' was created in Federico Fellini's 1960 film La Dolce Vita. Fellini had remembered a school friend whose fidgety movements and constant energy had earned him the nickname "paparazzo"("mosquito"), and the character of Signore Paparazzo, a news photographer, was born.

But the romantic associations of its inception have not endured. In the past 15 years, there can have been no species on earth more reviled than the paparazzi. Indeed, when Princess Diana's brother Earl Spencer gave the eulogy at her funeral he talked about her wish to leave the country because of "the newspapers, " and complained of a media plot to "bring her down."

"My explanation, " said Spencer, "is that genuine goodness is threatening to those at the opposite end of the moral spectrum. . . [Diana] was, in the end, the most hunted person of the modern age."

There - in that ferocious and most public of accusations - was the popular definition of the paparazzi: they are, in the public's eye at least, hunters.

The reference to photographers as hunters, poised to "shoot" their quarry, is not confined to the tense and highly competitive world of British celebrity journalism.

Even the Taoiseach, Bertie Ahern, drew the analogy at the recent launch of a book of photographs by leading snapper Pat Maxwell.

Unlikely as it seems, Ahern told the photographic doyens of the industry celebrating the publication of Stories Behind My News Pictures, that "A good photojournalist is, in some ways, like a good sniper - he waits patiently for that one shot that could become the snapshot of history."

The relationship between international celebrities and photographers was not always so poisonous.

Dave Bennett, who styles himself as a "showbiz photographer, " and stopped being an on-the-street hunter in 1994, remembers the 1980s, the golden early years of the British freelance celebrity photographer, as a more innocent time.

"The [paparazzi] industry has grown so much in the past 15 years that there are now dangerous numbers of people out there, " he says.

"When I was doing the paparazzi stuff, it was pretty jovial. The streets are very rough now."

PR Mark Borkowski says there has been a huge increase in "bounty hunters." "The number of oldschool paparazzi, those who have a good relationship with PRs and their clients, is very small indeed. Now we have people who seek a quarry.

These are people with no photographic skills. It's all about getting the picture. Digital technology has changed things for people. Now everyone can be a photographer."

What digital technology has also created, is the so-called Waparazzi - the camera-phone-toting Joes who take pictures of celebrities and send them to agencies. For most celebrities, this is wonderful news. Never have there been more opportunities to have one's knickerlessness displayed in so many formats. But for retiring types - such as Middleton - it is a precarious environment in which to protect one's privacy.

The relationship between the paparazzi and the privacy laws has always been a tricky one. Unlike in Germany, where photographers must ask permission from their subjects to take shots, the law here is fuzzy.

New privacy legislation was published last July by justice minister Michael McDowell. It broadly provides a defence against violations of or intrustions into an individual's privacy and therefore has major consequences for how photographers here do their work.

The courts will have to take into account the extent to which the journalist or photographer "engaged in surveillance, harassment or trespass on private property" - actions regularly carried out by the most eager paparazzi.

Most papers here have already drawn the line at gross infringement of celebrities' private lives, although the Star famously had to apologise to Bono a few years ago when they published a picture of him getting changed into his swimming togs on Killiney Beach.

The big question, however, is where public and private lives meet and this is something that will have to be tested in the courts.

"A private place, " says Stephen Abell of the British Press Complaints Commission, "is public or private property where there is a reasonable expectation of privacy. So, the commission has considered public places where one would have a reasonable expectation of privacy to be a church, inside a restaurant, or one's place of work." But not on the street?

"It depends on the circumstance.

For instance, the code talks about "persistent pursuit" being forbidden - so following someone is a contravention of the code."

Three of the most valuable news images of the past two years - Kate Moss taking cocaine; survivors finding their way out of Aldgate station on 7 July; and Saddam Hussein's execution - have been taken on mobile phones.

Here, Dublin Fianna Fáil councillor Liam Kelly became the first high-profile victim of the Irish 'waparazzi' last August when grainy pictures of him, which he denied depicted him snorting cocaine, appeared in the Sunday World last August.

It all depends on the nature of the photograph and the context in which it is taken - and, critically, what you do with it.

Dealing in paparazzi shots is now a highly sophisticated market in which every celebrity has a market worth. And it's not about the quality of the photograph. When the mobile-phone footage of Kate Moss snorting cocaine hit the front page of the Sunday Mirror, its rival, the Daily Mail, surmised that it had paid £150,000 for the images.

So, everybody has the potential to become seriously rich at the touch of a button. The paparazzi only continue to exist because they happen to be the most committed, and the most ruthless at collecting the images.