In the type of backup system I recommend (see Chapter 9 for complete details), your backup software begins by copying all your important files to some sort of external mediapreferably a hard drive. What counts as "important" is up to you; it could be everything on your disk, just the contents of your home folder or your Documents folder, or just the files you're actively working on. But at a minimum, you should make a daily copy of any files you could not re-create in a matter of minutes, such as your saved email, photographs, and any documents you've spent hours working on during the day.
I advocate performing additive incremental archives. This means that after your first full backup of all the files you want (creating the archive), each successive daily backup copies only those files that are new, or have changed, since the last time (that's the incremental part); and it keeps the previous copies of your files, so you can go back to an earlier version if you accidentally modify a file you shouldn't have (that's the additive part; it also means that files you delete on your hard disk remain in the archive).
In addition to automated daily backups, it never hurts to make extra copies of files you're actively working on. If you take a moment to drag such files to a network server or iDisk (or even make an extra copy on the same drive) whenever you stop to take a break, you'll add yet another layer of safety to your valuable data.
If you configured your backup software to run on a schedule, this happens automatically every day. You may, however, need to intervene in some cases, such as these:
Even if your backup software runs automatically, I recommend checking its logs regularly to make sure that it ran and that it backed up all the files you expected it to.