Boot a Mac from a CD / DVD
You can boot your Mac from a disk in the CD / DVD drive by inserting the disk into the drive and on system boot holding down the C key. Remember that booting from a CD or DVD is significantly slower than booting from a hard disk, so don’t worry if things are taking longer than usual to startup, it’s simply just slower to read from a spinning disc than the hard drive.
If you want to reinstall Mac OS X from a DVD reinstall disc or run Disk Utility from a disc on your main boot drive, you’ll need to do use this boot disc method. Of course, the CD or DVD must be bootable to begin with, whether that’s because it shipped with the Mac as a restore disc, or because you made your own boot DVD, either will work.
Something to keep in mind is that newer Macs don’t have SuperDrives built-in at all, and instead rely on Recovery partitions or Internet Recovery. Anything that shipped with OS X 10.7, 10.8, 10.9, will use one of these newer recovery methods, though they can continue to boot from a DVD if they have a SuperDrive or a DVD is attached through an external reader. Additionally, newer Mac models can boot from USB devices too, including external hard drives or USB flash drives.