Good backups are essential for every Mac user. Tools such as Apple’s Time Machine, included as part of OS X, make it easy to store multiple versions of every file from your computer on an external drive or an AirPort Time Capsule. And if you want the security of off-site backups without having to physically move drives around, an online backup provider such as CrashPlan is a good option.
But while both these forms of backup serve important purposes, I also recommend maintaining a clone (also known as a bootable duplicate)—a complete, identical copy of your startup volume, stored on an external drive in such a way that you can boot your Mac from it if necessary.
I’ve always recommended multiple backups. Using these apps will make the process relatively easy.