The fight-back against Napster has begun. Last week, rock band Metallica began a legal battle which the rest of the global recording industry will be watching very closely indeed.

Metallica said that the Napster computer software, produced by a company of the same name, was being used to flout copyright laws and deny Metallica's members the money they earn from record sales.

Napster (www.napster.com) grew out of the MP3 phenomenon. It is a file-sharing system allowing anyone to find files (of any sort, not just MP3s) and download them to their own computers.

MP3 is a term used to describe compressed audio files. Where a normal CD-quality song would need many megabytes of disk space to be stored on a computer, MP3s are smaller files which sound just as good as the original.

Not only do they take up less space on your computer, they are also much easier to transfer across the internet.

Of course, Napster has taken off like wildfire among people hunting for MP3 files of their favourite recording artists. Even though it's illegal to copy music and pass it on to other people in this way, thousands of people have started to do just that.

What Napster does - and what the record industry is horrified about - is massively increase the opportunity for people to make illegal copies.

You no longer need a friend to buy the album you want. You can search for it on the internet and download it from a total stranger, for free.

And unlike a cassette, it won't be a distorted, scratchy copy of a copy, but will sounds as perfect as the original did.

Metallica won the right to monitor Napster's traffic for a short period, and compiled a list of over 300,000 Napster users which it said had illegally downloaded its songs.

The list was presented to Napster's management with a short message: remove these users from your service or we'll take you to court. Napster, which so far has not had any significant income, had no choice but to comply. The 300,000 users were unceremoniously booted off the network.

So what happens next? More pop stars are queuing up to do the same, while others are stridently calling for an end to the "see you in court" attitude and want the industry to embrace electronic distribution.

Lars Ulrich, Metallica's drummer, accepts that Napster and MP3 will ultimately change the way music goes from artist to consumer.

But he and the rest of the industry want it done on their terms. A system like MP3 which allows unlimited copying of files is simply not a sustainable way to measure listeners and extract money from them.

The problem is, persuading the millions of MP3 users around the world to give up now and adopt some other format is going to be very hard indeed.