The literal meaning is:
Redundant Array of Independant Disks; a collection of storage disks with a controller (or controllers) to manage the storage of data on the disks.

What does this mean to you?
In short RAID will protect your business critical data guaranteeing that you will not have loss of service. With today’s technology you can implement a RAID solution and never have to shutdown your server in the event of disk failure.

The different RAID levels

RAID 0
RAID Level 0 is not redundant, hence does not truly fit the "RAID" acronym. In level 0, data is split across drives, resulting in higher data throughput. Since no redundant information is stored, performance is very good, but the failure of any disk in the array results in data loss. This level is commonly referred to as striping.

RAID 1
RAID Level 1 provides redundancy by writing all data to two or more drives. The performance of a level 1 array tends to be faster on reads and slower on writes compared to a single drive, but if either drive fails, no data is lost. This is a good entry-level redundant system, since only two drives are required; however, since one drive is used to store a duplicate of the data, the cost per megabyte is high. This level is commonly referred to as mirroring.

RAID 5
RAID Level 5 stripes data at a block level across several drives; parity is distributed among the drives. This can speed small writes in multiprocessing systems. Parity data must be skipped on each drive during reads this can reduce the performance for reads. Requires a minimum of three hard disk drives.

RAID 10
RAID 10 is a combination of RAID 0 and RAID 1. Unlike RAID 5, there is no need to calculate parity information. RAID 10 disk arrays offer good performance and data security.

Summary:

• RAID 0 is the fastest and most efficient array type but offers no fault-tolerance.
• RAID 1 is the array of choice for performance-critical, fault-tolerant environments. In addition, RAID-1 is the only choice for fault-tolerance if no more than two drives are desired.
• RAID 5 is the best choices in multi-user environments which are not write performance sensitive. However, at least three and more typically five drives are required for RAID-5 arrays.
• RAID 10 offers all the benefits of RAID-0 and RAID-1 and is typically used for situations where high sequential write performance is required.