OSX has a Recovery function built in which allows you to repair disks or reinstall OS X without a DVD/ CD Rom. Some
of the new models such as the Macbook Air don’t have a DVD player. So how can you reinstall OS X without a DVD drive???
With the OS X Recovery Disk Assistant on a USB or external drive you can format the Macs hard drive and reinstall Lion or Mountain Lion. You can also repair the disk using Disk Utility, restore a backup from Time Machine or simply browse the internet with Safari.