Some common ways to have a system backup are as follows-

a) Tapes can be used as back up storage devices.

b) Optical devices such as -i]DVD's ii]CD's
can be used too as data storage devices.

c) Flash memories (USB Drives, Portable Hard Drives) another device which can also be used for storage for the data that we need to have a backup.

d) Online services, are one another effective and hitech way to have you data backups.These are paid services provide by some sites specially working and providing such services only.For example Xdrive, allow users to upload their files to a server for safe keeping. Although it may be convenient to have the data available wherever an internet connection is available, there are a few limitations.
i use the last three ways of back up-ing. and for me using the usb is the most reliable one among all of them. because you can access to your files even without the net. and the fact that sometimes cd's are not readable by some.