There are many different RAID types, each having their own advantages and disadvantages but each is designed to safe guard your data and speed up data transfer times.
The most common types we see are RAID 0, RAID 1 and RAID 5 but there are others.
RAID 0 is where the data is striped between two hard disk drives. This means that if one hard disk drive fails and is not recoverable, you have essentially lost half of your data; it does however improve performance.
RAID 1 is a mirrored pair, which is where the data is written simultaneously to both hard disk drives. This provides greater safe-guarding against data loss as you will always have a copy if one hard disk drive fails
RAID 5 is where data is spread over a series of hard disk drives, usually 3 or more, and can tolerate a single hard disk drive failure without the user being aware of any data loss. Until the failed hard disk drive is replaced however there will be a noticeable loss in performance; 2 or more hard disk drives failing can result in serious data loss.