RAID is short for "Redundant Arrayof Inexpensive(or Independent) Disks", originally opposed to "Single Large Expensive Disk" (SLED). In a RAID, data is distributed between several physical disks (called "Member Disks") to provide additional reliability and/or speed increase. There are several "RAID levels" different in data placement algorithms (see below). The array of disks is presented to the operating system as a single large disk so that operations with RAID array do not differ from the operations on the regular device with respect to the file storage (and filesystem drivers in general).
RAID is implemented either by the driver (this is called "software RAID") or by the special hardware controller ("hardware RAID"). Both Microsoft Windows NT and most flavors of Unix/Linux provide their software RAID implementations. Microsoft Windows 9x series operating systems do not feature a software RAID drivers, but still can be used with a hardware RAID controllers (because of the controller transparency).
RAID does not provide perfect reliability:
RAID, no matter how redundant, does not substitute proper and regular backups.
Consider the following points:
JBOD (Just a Bunch Of Disks), also called "Span"/"Spanned Volume":
This is not a RAID in the strict sense, because JBOD does not provide any redundancy. If any one drive in the JBOD-type array fails, the whole array fails and all data on it is lost. Typical usage of JBOD layout is just to create a disk of larger capacity by merging two or more smaller disks. This is only practical if the disks have different capacity. For the disks of equal capacity, RAID 0 is better because it provides the same capacity increase, the same non-redundant layout with no disk space overhead and features faster read/write speed in typical applications. JBOD can provide speed increase if two operations are requested simultaneously on data blocks that are stored on different drives, but this is relatively rare situation (let's say for example that read of blocks D1, D2, D3 and D11, D12 is requested simultaneously; in such a case two requests can be performed in parallel, increasing overall I/O speed).
Minimum of two disks is required for a hardware JBOD. Minimum of one disk is required for a software JBOD (Windows NT/2000/2003 "Spanned volume", which allows volume to occupy nonadjacent regions of the same physical disk). There is no disk space overhead. The following exception may or may not apply in your case: hardware RAID controller may support a single-disk JBOD configuration - this is just a trick to allow a single drive to be attached to the controller, without actually RAIDing anything. The same applies to RAID0 consisting of 1 member disk.
RAID 0, also called "Stripe Set":
This one again is a "non-redundant" RAID. RAID0 fails if any member disk of the array fails. Major benefit of RAID 0 is that it provides N times I/O throughput increase for N-disk configuration. The read/write requests are scattered evenly across member disks, so they can be executed in parallel. For example, if write of data blocks D1 through D6 is requested odd blocks (D1, D3, D5) should be written to Disk 1 and even blocks (D2, D4, D6) should be written to Disk 2, which doubles the overall operation speed.
Minimum of two disks is required for a RAID 0 setup. There is no disk space overhead associated with RAID 0 volume. The following exception may or may not apply in your case: hardware RAID controller may support a single-disk RAID0 configuration - this is just a trick to allow a single drive to be attached to the controller, without actually RAIDing anything. The same applies to JBOD consisting of 1 member disk.
RAID 1, also called "Mirror":
In a mirrored volume, two exact copies of data are written to two member disks. Thus, a "shadow" disk is an exact copy of its "primary" disk. This layout can tolerate loss of any single disk (read requests will be satisfied from the functional disk). Mirrored volume features twice the read speed of a single disk: when requested to read data blocks 1 through 6, the mirror routes odd blocks (D1, D3, D5) to be read from Disk 1 and even blocks (D2, D4, D6) from Disk 2 so that each disk does half of the job. Write speed is not improved because all copies of a mirror need updating.
It is possible to have more than two disks in the mirrored set (e.g. three-disk configuration - "primary" with two "shadows"), but such a setup is extremely rare due to the high disk space overhead (200% overhead in three-disk configuration).
Exactly two disks are required for a RAID 1 volume. RAID 1 layout imposes 100% storage space overhead.
RAID 5, also called "Stripe Set with parity":
RAID 5 utilizes a parity function to provide redundancy and data reconstruction. Typically, an "exclusive OR" ("XOR") binary function is used to compute parity for a given "row" of the array. Anyway, the parity is computed as a function of several data blocks P=P(D1, D2, ... DN-1) for N disk layout. In case of a single drive failure, the inverse function is used to compute data from the remaining data blocks and parity block.
Let's say for example that the Disk 3 fails in configuration illustrated below.
During normal operation, read speed gain is (N-1) times, because requests will be evenly routed to N-1 disks (parity read is not needed during normal operations). Write procedure is more complicated, and actually imposes some speed penalty. Let's say we need to write block D1. We also need to update its corresponding parity block P1,2. There are two ways to accomplish this:
Both of these ways require at least one read operation as the overhead. This read operation can not be parallelized in any way with its corresponding write operation, so write speed should decrease (by the factor of two, assuming equal read and write speed). Most current implementations mitigate this effect by maintaining the entire "row" (D1, D2 and D3) in the cache.
Minimum of three disks is required to implement RAID5. Storage space overhead equals the capacity of a single member disk and does not depend on the number of disks.
RAID 0+1, also called "Mirrored Stripe Set":
This layout combines speed efficiency of the RAID 0 (stripe set) with a fault tolerance of RAID 1 (mirror). Its only drawback is the 100% disk space overhead.
For N-disk configuration (with two stripe sets with N/2 disks each)
The RAID 0+1 array can tolerate a loss of any single disk. Additionally, it can handle half of the dual failures in the four-disk configuration (e.g. if disks 1 and 2 fail in configuration illustrated below, the array will still be online albeit degraded to a stripe set).
Minimum of a four disks is required for a RAID 0+1 volume, with a 100% storage space overhead.
Other RAID layouts:
RAID 3 and RAID 4 are similar to RAID 5 but use dedicated disk to store parity information. This disk becomes a bottleneck during write.
RAID 6 is similar to the RAID 5 but uses two different parity functions to maintain redundancy. RAID6 can tolerate a dual failure (simultaneous loss of two drives). RAID6 is useful in high-capacity systems when the rebuild of a RAID5 would take a long time and there is a significant probability that another drive will fail before the rebuild is done, thus causing a loss of the array.