Boot your 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.
If you want to reinstall Mac OS X or run Disk Utility on your main boot drive, you’ll need to do this. Now this isn’t really an advanced tip by any means, but amusingly enough I actually forgot how to do this (I was holding D, oops) and I’m a long time Mac user!