Making regular backups is one of the most important and, at the same time one of the most neglected aspects of computer ownership. Backing up your data should be at the top of your computer maintenance list, right next to virus protection. Without data backup or virus protection, you are running the risk of losing your data. And it will happen, don’t think that you don’t have to worry about it.
Three months ago my iMac began behaving erratically. After much experimentation the cause became quite clear as my internal hard drive physically crashed rendering it all but useless and unrecoverable. All my work, my photos, 10000+ music files, bookmarks, feeds - everything was gone. The iMac was still under Applecare so replacing the drive was no problem and took about a day. They even upgraded me from 350gb to 500gb at no additional charge. Yay, Apple!
In an uncharacteristic stroke of brilliance I had installed Time Machine about a year ago and literally within minutes I had restored my new hard drive to the exact state it was in the day before the crash - losing no data at all. Learn from this.
On my desktop iMac, I use Apple's proprietary Time Machine software to back up to a LaCie d2 Quadra drive. On my Macbook Air I back up to the cloud using Backblaze.