The medium chosen for backing up data depends on the volume of data, and also the nature of the data.
CD: The most common type of media backup used is CD-R and CD-RW. The first one is cheap but you will use a lot of them for a full backup because they hold only 650 MB of data. The second is more expensive but has the ability to be written on again and again. It is a good choice for obtaining backups, and is one of the favourite choices in both domestic and corporate spheres. It is cheap, portable and compatible with several file formats.
DVD: DVD is a good choice for backing up data. It may not be as cheap as the CD, but offers better quality, and has more space. It can hold around 4GB of data. DVD supposed to be last longer than CD.
USB Devices: These are the new favourites all over the world. The storage space of flash drives is constantly increasing. The common storage space of USB device is 4GB, 8GB and 16GB now. And the price of USB device drops rapidly. It does not require a separate software or booting up of the machine to start functioning, and it is very convenient to share the data stored on them.
External Hard Drives: For larger data storage requirements, external hard drives are excellent solutions. Arrays or stacks of hard drives are available for corporate purposes. Several hundreds gigabytes (e.g. 320GB or 500GB) is common size for hard-drives. You can even find 1 TeraBytes hard drives now.
Online and Offline Storage Spaces: These are “spaces” provided by professional firms, and are extremely secure for sensitive data storage.
But you need to rely on the provider services. The quality and security are all depends on the services provided by the company.