Tiger Woods is a sporting phenomenon. In winning his fourth consecutive major championship at Augusta, Georgia, the 25-year-old golfer achieved a feat that few previously thought possible.

But Woods is also a marketing phenomenon. Last year, according to figures compiled by the US business magazine Forbes, he was one of two athletes whose earnings topped $50 million. The other was Michael Schumacher, the Formula One racing driver. Only about $9 million of Woods's $53 million earnings was said to have come directly from playing golf.

It is easy to understand why a sports goods manufacturer might wish to see its name linked with Woods's extraordinary prowess on the golf course. One of his main sponsorship deals is with Nike, the US sports equipment company. He also has contracts with non-sporting companies including General Motors, Rolex and American Express, and is thought to be close to signing another with Walt Disney.

What drives companies to pay vast sums to associate their brands with sports celebrities who have little to do with the products being promoted? And have these stars now achieved such marketing potential that their names have become brands in themselves?

The answer to the first question is not particularly complex. Fame sells products and the traditional way of building it is through advertising. But a handy shortcut is to hitch your product to someone who is already famous.

"Celebrity endorsement gives you faster recognition and a faster development of personality for your products by borrowing some of that recognition and personality from someone who already possesses it," says Alex Batchelor, managing director of Interbrand, a brand consultancy.

As Mr Batchelor points out, this has been going on for almost as long as celebrities have existed. What has changed is that globalisation has raised the stakes - and nowhere more than in sport. This is partly because of the technological breakthroughs that have made it possible for viewers across the globe to witness the feats of the leading sportsmen and women in a way that was not open to previous generations.

"Because of the power of the media, our awareness of the stars of sport is much greater," says Guy Kinnings, head of golf clients at IMG Europe, a sports management company.

L'Oreal, the French cosmetics group, has contracts with a number of sports stars including Michael Schumacher, the footballers Emmanuel Petit and Paolo Maldini, and the tennis player Alex Corretja. It says it uses them because of "their spirit of victory". Schumacher's association with technology is also said to chime well with "the technology of L'Oreal products".

But many sponsors are interested in the personalities of sports stars even if they are not victors. "Often it is not who plays the best. It is who does it with a certain style," says IMG's Mr Kinnings. "Arnold Palmer (the former top golfer) has endorsed most products because he has a fantastic image."

Mr Kinnings says Woods has particular attractions for sponsors because of his age. "He brought a whole new approach to golf that introduced a whole new generation of people to golf that hadn't been there before. The demographics of TV viewers watching golf has changed since he came on the scene. He introduced more youngsters."

This helps to explain Disney's interest, says Peter Diamond, director of market planning at Wolff Olins, a London-based brand consultancy.

"Disney says `OK, we are all about fun. Who can we use to make that brand image stronger'?" he says. "Tiger Woods, based on his current brand persona, definitely supports that and embodies it."

Such comments are borne out by the viewing figures for the final round of the Masters tournament, all or part of which is estimated to have been watched by 40.6 million people - just behind the 43 million who watched Woods win the Masters for the first time in 1997. "Age span is something that is extraordinary with him. Whether you are 10 years old or 70 he really made a difference," says an American Express executive.

It is this kind of pulling power, and the marketing muscle that comes with it, that raises the question of whether sports stars are starting to become brands in their own right. But many marketing professionals think a brand is more than that. In marketing-speak, a brand stands for a set of values that distinguishes a product from others in the same category and enables the owner to charge a higher price for it. Sports stars almost qualify on this definition: they are associated with certain attributes and their names command premium prices. But they still fall short of being brands in other respects. A difference between brands and most sports stars is that brand names stand for things that are sold, while stars act only as endorsers of products. To become brands in their own right, stars would have to move beyond simply recommending other people's wares and start to market their own.

Simon Williams, a director at Enterprise IG, a London-based brand consultancy, says there is an elite of global stars who command enough emotional resonance to be categorised as having "a brand-type relationship" with their audiences.

"My idea of a brand is that it's something that lives beyond the actual flesh and blood of human beings," says Mr Williams.