Protecting our valuable data- pictures, videos, documents...is one of the most important, required tasks of owning and using a computer and not having it drive you nuts! It's also kind of boring and time-consuming and that's why folks often overlook it's importance. IT IS VITAL!
Several years ago when I was newbie, my only external hard drive full of videos failed in the middle of a paid editing project. It was in that miserable moment I realized how stupid I felt about not backing up those files. My excuse was I didn't have enough money for another hard drive! That kind of thinking sinks ships.
The only thing I can add here that may not have been covered is this: If you're going to backup iPhoto (for instance) onto an external hard drive and then delete the original from your system drive, what happens if the ext hard drive craps out on you while you make a backup of it to a second ext hard drive? It's not likely, but it happens!
The safe way is to copy iPhoto to both ext hard drives first, then delete the original from your system drive.
Some people may complain they don't have enough money to buy an additional hard drive and I get that. The truth is if you have anything on your computer, such as photos, and they mean A LOT to you and you don't have them backed up, you will truly be bummed, furious, flabbergasted, etc. if they disappear forever. If you can't afford external hard drives, you can always backup the original images in iPhoto (go to Documents, then Pictures, find iPhoto Library, don't open it, but select Show Package Contents and your original images are in the Masters folder) to CD's or DVD's.
If you haven't lost all your original files from a hard drive (yet), you may be inclined to say you'll get around to it soon. Guaranteed when it happens (and it eventually will) you'll be having one of those "I knew I shoulda done that!" moments. You'll notice I've mentioned this a few times here. Just can't overstate the importance of backup. Today hard drives are super cheap and keep getting cheaper.