Let me tell you about RAID 5. RAID 5 is basically a way of storing electronic data; it takes a set of five hard disks and ties them together. From the outside, a RAID array behaves as if it were a single disk. However, the data is distributed across all five disks - and so is the parity data, which is basically a record of what each disk has on it. So, each disk has part of the data, plus a record of what data the other disks have.
The reason for setting it up this way is that it helps prevent data from being lost. If one of the hard disks crashes, there's still enough data spread across the other four that the overall array can continue to function uninterrupted. Even better, if you pull out the dead hardware and replace it with a new drive, the data on the other four disks can be used to "fill in the blanks" for the new arrival. This takes a little time, but once it's done you're back to full RAID 5 redundancy... and in the meantime, your array continues to operate as a data storage device.
As you can probably imagine, an array like this this is a considerable improvement over a single hard disk by itself; if a single disk crashes, everything on it is probably gone. The only real problem with a RAID 5 array is that if you happen to have one disk crash, and you remove it and replace it, and then you have a second hard disk crash while the system is in the process of rebuilding the data on the first disk... well... "KABLOOEY!" as Calvin would say.
Go on. Ask me how my day is going.