RAID(redundant array of inexpensive disk) is a technology through which we can use the multiple hard disk in a single system.
in case one hard disk is crash then to another hard disk recover the data

RAID stands for Redundant Array of Independent Disks and it
basically involves combining two or more drives together to
improve the performance and the fault tolerance. Combining
two or more drives together also offers improved reliability
and larger data volume sizes. A RAID distributes the data
across several disks and the operating system considers this
array as a single disk.
RAID Levels:
Several different arrangements are possible and different
standard schemes have evolved which represent a set of
trade-offs between capacity, speed and protection against
the data loss.
Some of the common RAID levels are -
Level 0
Level 1
Level 2
Level 3
Level 4
Level 5
The above standard RAID levels can be combined together in
different ways to create Nested RAID Levels which offer
improved performance. Some of the known Nested RAID Levels are:
RAID 0+1
RAID 1+0
RAID 3+0
RAID 0+3
RAID 10+0
RAID 5+0
RAID 6+0

RAID(redundant array of inexpensive disk) is a technology 
through which we can use the multiple hard disk in a single 
system.And also,in case one hard disk is crash then to 
another hard disk recover the data it basically involves 
combining two or more drives together to improve the 
performance and the fault tolerance.