A timely reminder: having had another client lose all her data this week due to hard drive failure and not backing up, may I suggest once again that everyone run a Time Machine backup. If the drive is under warranty it will be replaced but YOUR DATA IS YOUR OWN RESPONSIBILITY!
Time Machine is the built-in software that backs up the entire computer to an external Hard Drive. It is very simple to set up and run, but IT ONLY WORKS IF THE DRIVE IS PLUGGED IN!
This is no issue on an iMac desktop, but obviously on a laptop you need to pop it in regularly because it is unplugged when you move around. "You are only as safe as your last backup". I have 2 running to be sure....
There is also an Airport Time Capsule available which backs up wirelessly so you just set and forget. As soon as the laptop gets in range it backs up.