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There are several different types of optical disc, although they all look pretty much the same.

CD (Compact Disc)

Optical discs that use the same technology as music CDs. They store up to 700MB of data. CDs can be used for multimediaapplications such as encyclopaedias and can store pictures, sounds and video clips or anything else that will fit.

There are several formats on the market, such as:

• CD-ROM - read only, the data is written to them before they are sold.
• CD-R - meaning CD-Recordable, the user can write data to the CD once or fill it over time using multi-session (writing to the same disc on separate occasions).
• CD-RW - meaning CD-ReWritable, the CD can be written and re-written to. Unlike multi-session discs, existing data can be overwritten.

DVD (Digital Versatile Disc)

DVDs are the same physical size as CDs but hold much more data - a single sided disc can hold up to 4.7 GB. DVDs are commonly used for storing video so you will often see them measured in minutes, eg 4.7 GB = 120 minutes.

There are several formats on the market, such as:

• DVD-ROMs - read only, the data is written to them before they are sold.
• DVD-R - meaning DVD-Recordable, the user can write data to the DVD once or fill it over time using multi-session.
• DVD-RW - meaning DVD-ReWritable, the DVD can be written and re-written to. Unlike multi-session discs, existing data can be overwritten.

CD/DVD drives

To read from and write data to CDs and DVDs you will need a suitable drive. CD/DVD drives are able to:

• read all CD and DVD formats
• write to CD-R, CD-RW, DVD-R and DVD-RW

Data is written to and read from the discs using a laser.