$RAID 
Stands for Redundant Array of Inexpensive Disks. This is a device that consists of a set of hard disk drives and is used in high-performance PCs or in network servers. The idea is that instead of using one big disk drive, it's more reliable to use eight cheap smaller drives and add one spare in case of a fault. The whole lot are packaged in a case. RAIDs are all the rage at the moment, so you will see a lot of advertisements with this term.