Hi,
Everytime you switch on a computer,it boots into (starts a operating system or other) so that you can use your PC in a sensible manner(by loading it into the main memory ) .In this case your operating system(like windows XP,98 etc) are installed on your hard disk(which is a storage media).
For instance, when you are unable to boot from your hard disk (or bcoz of any other reason ) , you boot from other storage media (such as CD ,USB etc)
A bootable CD is one , which loads an Operating System,or any other s/w into your main memory(also called RAM) so that you can sensibly use your computer system without booting from your hard disk.
You have to use such techniques when...
1)You want to recover your crashed disk/data
2)You want to remove boot sector viruses
3)You want to format your hard disk
4)You want to install another OS
