A bootable CD is the media which runs automatically once inserted, and its main use lies in installing an Operating System
etc. An Operating System
is the platform between the user and the kernel of the system that is instrumental in running a computer system. Without any kind of Operating System, no system can run. We are very familiar with Windows XP CDs which, after inserting and restarting of the system, automatically starts to load itself. Windows 7 is similar. Apart from installing, a bootable CD can be used to repair any crashed operating system, and more utility can be performed depending upon the type of CD. Some popular external bootable CDs are Hiren Boot CD and BartPE, which are used by professionals for various reasons related to system maintenance.
Some of the more popular ones are Ultimate Boot CD (http://www.ultimatebootcd.com/),
and Hiren's Boot CD (http://www.hiren.info/pages/bootcd).