Redundant Array of Inexpensive/Independent Disks (RAID) is a hardware virtualisation technique that creates a single logical storage unit from multiple hard disks, providing a performance boost, fault tolerance, or both.
There are many different types (levels) of RAID. The most common ones are RAID 0, RAID 1 and RAID 10.
RAID 0
RAID 0 splits data and stores the parts across multiple hard disks, without redundancy or fault tolerance. The failure of a single drive will result in total data loss. This setup is mainly used to increase throughput. Theoretically the throughput of RAID 0 can be as high as the combined throughput of all the drives in the array.
RAID 1
RAID 1 is known as a mirroring setup. A copy of the data is stored independently on each drive in the array, providing fault tolerance. As long as one drive in the array still functions correctly, the array remains in working order. As each drive can handle read operations independently, RAID 1 provides improved read throughput. The write speed is determined by the slowest disk in the array.
RAID 10
Often noted as RAID 1+0, RAID 10 is a nested setup of RAID 1 and RAID 0 - a striped mirror of hard disks. It usually requires at least 4 disks to provide all the benefits of the setup. RAID 10 provides the best throughput (apart from RAID 0), redundancy and fault tolerance among all the RAID levels.