Graffiti is viewed by many with unalloyed revulsion, and I find the feeling rising up in myself when confronted with obscene phrases and pictures scrawled on playground slides and climbing frames, or names crudely etched into bus and train windows.

They are, however, part of a spectrum in graffiti that extends to elaborate, carefully planned, colourful, largescale murals which beautify rather than degrade their surroundings. A few, at least, of the kids "tagging" trains have, inadvertently or not, entered training for a far more serious pursuit.

Most art is unadmittedly competitive; graffiti is nakedly so. Writers vie for prominence of their works, their size, complexity, technique and above all their ubiquity. Elaborate etiquette regulates this rivalry, and competition is joined by overwriting a rival's work. When I accompanied a TV team to watch the well-known writer, Prime, produce a work on a quasi-official site, the most interesting and shocking act was his first - taking a large roller to the painting already there, entirely blotting it out.

If convention governs the terms of rivalry and respect between writers, it also quite rigidly governs the look of graffiti. Simple or elaborate, graffiti is founded on the name. It is a do-it-yourself version of that larger vandalism of the environment, advertising.

The tag is a logo, publicising the anonymous and the powerless. Its large, multi-coloured variants sparkle with gold and silver paint, and use every visual trick to throw themselves into movement in emulation of the flashy, animated logos that open movies. Ownership of a tag is as jealously protected as that of a brand, with works sometimes bearing a wishful copyright sign. The competition between graffiti writers over size, prominence and distribution is a direct reflection of that between advertisers. Given this affinity, it is no surprise that advertisers have tried to use graffiti, offering writers inducements to work for them (London's infamous graffiti writer Banksy has had plenty of offers), or simply doing it themselves, especially using stencils.

Most large graffiti pieces are made illegally, at night, in a hurry, from designs already worked out on paper, and with a limited range of colours. While graffiti crews who work collectively do travel, sometimes internationally, much of the work is local. The inhabitants of an area decorate an environment over which they usually exercise no power. They generally do so in places that are run-down and publicly owned.

The fascination of these pieces lies not in the designs alone but in their interaction with a particular place (the art world calls this "site specificity"), with other works, adjacent or over-written, and with the surface on which they sit - the texture of brick, concrete, wood or metal, new or old, dry or damp. All these pieces will decay, be buried under the marks of rivals or subsequent generations, be removed by the authorities or demolished with the walls on which they are painted. Their life is changeable, generally brief and, in the unremunerated sacrifice of labour for selfexpression, poignant.

Sometimes graffiti is brought indoors and paraded as gallery art. There was a fashion for this in the 1980s, recently revived, of which the most successful exponents were Keith Haring and Jean-Michel Basquiat. Both of these artists had made graffiti but, carrying with them a few of its techniques, evolved highly individual and unconventional styles of painting. It is more troublesome to get graffiti writers to decorate a canvas as they would a wall.

The resulting paintings are done in a more leisurely manner, with unlimited resources on smooth expanses of canvas by artists who are no longer competing for space or altering a particular environment. All urgency, much of the interest and the very point of street graffiti is lost. Besides, the ethos of graffiti - to make temporary and sometimes unwanted gifts to a public - is at odds with the commercial imperative to make permanent, moveable works for sale.

The current art-world crush on graffiti in New York is connected to the concerted effort, initiated by the former Mayor Rudolph Giuliani, to drive graffiti from the streets and particularly-from the trains. Dedicated graffiticrews had risked serious injury and arrest to decorate these trains, infiltrating the sidings and on occasion decorating carriages or even entire trains from top to bottom with extraordinary murals that would for a single day travel through neglected neighbourhoods, often to considerable public acclaim, before their erasure by the cleaners.

Denied their usual environment, graffiti writers end up in galleries or making legal pieces advertising trainers and soft drinks. (There is an opportunity to see examples of this history in a survey of 30 years of New York graffiti in the current East End exhibition, Bombers, at Jeffrey Charles Gallery and Whitechapel Project Space.)

Just because graffiti tends not to live comfortably in the gallery does not mean it is not art. I was once called as an expert witness in the trial of a graffiti writer that, fortunately, never came to court. He had been found decorating the walls of an abandoned swimming pool, long used as a graffiti site, and the local council seemed determined to make an example of him. The defence was to be made on human rights grounds: to argue that he was pursuing selfexpression through making art, and that the state had no right to interfere, especially if it involved no damage to property. There is no ignoring the place of graffiti in popular and commercial culture, and no going back to stringent prohibition.