What is RAID? - Redundant Array of Inexpensive/Independent disks.
It is the technique in which we use multiple physical hard-drives together as a single logical hard-drive.

Standard RAID Levels(RAID Types) in operating system

RAID 0: It stripes the data and stores it in the available disks. Striping can be at bit/byte/block level.

RAID 1: It uses mirroring. It makes copies of all the available data and increases reliability

RAID 2: This uses Error Correcting Code(ECC) like hamming code. More than 2 bits are used to store the ECC so minimum 3 dedicated disks are required to store the ECC.

RAID 3:This is bit wise separation of data.Instead of ECC it uses Parity. Only 1 bit is used so only one dedicated disk is required. It increases data transfer rate but doesn't increase number of accesses.

RAID 4: It is similar to RAID 3 but here data is divided block wise.It increases number of accesses but no improvement in performance.

RAID 5:Instead of using dedicated disk for storing parity information it divides the parity info across all the disks. It uses 1 bit for parity.

RAID 6: It is similar to RAID 5 but it saves 2 bits of parity information.