RAID stands for Redundant Array of Inexpensive Disks. A RAID system consists of two or more disks working in parallel. They appear as one drive to the user, and offer enhanced performance or security (or both).
The software to perform the RAID-functionality and control the hard disks can either be located on a separate controller card (a hardware RAID controller) or it can simply be a driver. Both Windows NT 4 and 2000 include a software RAID solution. Hardware RAID controllers cost more than pure software but they also offer better performance.
Most RAID-systems are based on SCSI, although implementations using IDE disks or FC (fibre channel) disks also exist. There are even systems that use IDE disks internally but that have a SCSI-interface for the host system.
There are different RAID levels, each suiting specific situations. RAID levels are not standardized by an industry group. This explains why companies are sometimes creative and come up with their own unique implementations.
Sometimes disks in a RAID system are defined as JBOD, which stands for 'just a bunch of disks'. This means that those disks do not use a specific RAID level and are used as if they were stand-alone disks. This is often done for disks that contain swap files or spooling data.
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