It is short for Redundant Array of Independent Disks. In the modern world, data is more important than the hardware on which it is stored. If you don't believe this, consider the millions of dollars Amazon.com earns each day through the transactions it does on its web site. What is the value of 1-20 hard disks compared to million of dollars a day in sales? Given this, most companies decided to store their data redundantly on many disks, a RAID, so that the failure of one or even a few disk does not lose the information.