RAID..Redundant Array of Inexpensive Drives..instead of having 1 HD, you have 2-whatever number. The data is written in several different ways (levels 1-5..but I'm sure there's more than that since I stuck my head in a Server..;); for example, it can write 2 bits of every byte to each of 4 hard drives. Then, if one drive dies, you can perform a Boolean algebra operation to get back the missing data (nothing to do with Bryce, dear friends).
To lose a raid array, most likely more than 1 drive died at the same time; rare, but it happens. What they have to do is replace the affected drives, reload the OS, and put back the data from the last backup done. Depending on their backup schedule, the data between the last backup and the failure is lost. Well, that's the short answer..;) Wow, I actually get to talk about something I know about for once..;)

I wish I'd said that.. The Staircase Wit

anahl nathrak uth vas betude doth yel dyenvey..;)