Remember the old floppy disk drives? They came in two varieties, 5.25 and 3.5". They were standard on most computers for 20 years and let you easily backup a file and take it with you where ever you went. The first computer to do away with the floppy drive was the original Bondi Blue iMac. The world thought Apple had gone made.
Over the nearly eight years since the original iMac was released we have come along way and now most computers ship without floppy drives. They have been replaced by something far more durable and easy to use - the USB flash drive or thumb drive. If you don't have a USB flash drive I suggest you get one and attach it to your keyring so it's with you all the time.
Flash drives are small pieces of solid state memory that you simply insert into the USB slot on a Mac or PC and copy your files to. Flash drives aren't new, I got my first one around 4 years ago. However as memory prices have fallen you can now get a 512Mb flash drive for around $20. As a comparison, you can fit about 500 old floppy disks worth of information on a 512Mb flash drive. And 512Mb is the minimum you should buy. I've recently seen 4Gb and 8Gb drives which will be the standard in about a year.
Flash memory is also quickly replacing the the standard hard drive as a better storage medium. There are no moving parts, no noise, and flash memory is a lot more durable. That means the laptop you buy in 3 years time could be half the size and have a battery life of more than six hours.