One problem that we, digital phtographers, are facing is the archiving of our beloved images. The backup is another important activity that should became part of the daily routine or workgflow and is strictly related to archiving.
The difference is that archiving is made according your choosen way, and may not be daily (but most serious photographers do), backup MUST be a daily activity. I never stop to recommend that.
To archive images people use mostly DVD (now blue ray will became the coolest choice) or an external hard disk. Pro's and serious photographers usually archive images on two or three different support and often store them in different physical places.
For backup, the most practical choice are these portable external Hard drives, that come in size from 60 to 500 gigabytes. But already exist at least one brand of my knowledge with a portable HD of a Terabyte.
One last thing I'd like to mention, is the recovery of lost images. One day happened to me to erase accidentally all images from a CF card. I thought the photos were on my Hard drive, but they weren't... panic, horror, shock!
After some deep breathing to calm down, I started to recall about some kind of software for recover of erased data. Internet is great for this and in a while I came up with this great tool (a freeware), that allow you to mostly recover your images (or any digital data) that reside on your card or your HD.
One tip: to recover files deleted by mistake is necessary you do it at once, the more fresh is the recovery after deletion the most chance of success.