'So my first question is: what is a CD-Rom?'

They're not just for storing Delia Smith recipes and playing games

Cecile Darroll works as an environmental protection team leader in a London borough. She, like many people, is confused about computers and especially about CD-Roms. Steve Homer invited her for a chat.

Cecile: We have a CD-Rom at work, but only one person knows how to use it. So the first question is: what is a CD-Rom?

Steve: A CD-Rom is a disc that stores data on it. That data can be anything. It can be a program like Microsoft Word, or photographs, or moving images, or it can be a lot of text: it could be all the legal precedents relating to your work.

Cecile: How does it differ from the hard disk in my PC?

Steve: In a way, it doesn't. The CD-Rom stores the data in exactly the same format as the other disks - the bits and bytes are exactly the same.

Physically, however, a CD-Rom is very different to a hard drive, which uses magnetic recording in the same way as a video or cassette recorder, except the data is recorded on rapidly spinning discs, not on tape. A CD-Rom stores data in the same way as an audio CD: through tiny pits on the disc and a laser shining on them to read the data.

Cecile: So is it quicker then?

Steve: No, but it is a lot cheaper. A typical hard disk sold today will store 500Mb to 1,000Mb. A CD-Rom will store over 600Mb. The difference is that a hard disk will cost between pounds 150-pounds 400, while a CD-Rom disc will cost perhaps a 10th of that. But you will need a CD-Rom drive to play CD-Roms and that will set you back about pounds 100.

Cecile: That still sounds quite cheap.

Steve: Ah, but what you can't do with a CD-Rom is record data. You can't store your own files, as you can on a hard disk, unless you have a recordable CD-Rom drive.

Cecile: How much do they cost?

Steve: Anything from pounds 700 to pounds 1,500. But you also have to buy special blank discs, which cost about pounds 5. Then you can build your own library of files.

Cecile: Why do people have CD-Rom machines at home? To have Delia Smith recipes and games and so on?

Steve: Yes. Most people buy CD-Rom discs for games, encyclopaedias, reference works and all sorts of software titles that are really rather fun.

Cecile: So did games come out years ago on CD-Rom? And when were CD- Roms invented?

Steve: Games used to be released on floppy disks.

Cecile: They went out with the Ark.

Steve: No, they are still useful for storing data and they are fine for most computing jobs. But you can only store 1.4Mb compared to over 600Mb on a CD-Rom, which means you can include more photographs, sound and movies, all of which take up a lot of disk space. As to age, CD-Roms have been around for about 10 years but they have only taken off in the past two or three years.

Cecile: Why?

Steve: It's a bit of a chicken-and-egg situation. The discs were expensive and the drives, too. As the two got cheaper, more people bought them, and as more people bought them they got even cheaper.

Cecile: So, is there anything in the pipeline to replace it?

Steve: Well, at the end of this year or the beginning of next, a new type of disc capable of storing 13 times as much data as a CD-Rom goes on the market.

The discs will probably be very expensive to start off with so you should not be put off buying a CD-Rom drive if you need one now. But these new discs are definitely the thing of the future and in a few years' time everyone will be using these so-called DVD-Rom devices. The good news is you will still be able to play your existing CD-Roms on the new drives.