There are various standard RAID levels from RAID 0 to RAID 6.
RAID 0: stripes the data across a set of drives but if you lose a drive you lose everything
RAID 1: mirrors the data to another disk usually in mirrored pairs of two
RAID 3: stripes the data and has a drive just for parity
RAID 5: distributes data and parity across the drives in case you lose one
RAID 6: distributes data and doubles the parity allowing for a two drive failure
The most commonly used configuration is RAID 5. In order to understand what RAID is you need to understand how the hard drives work in your computer or externally in an enclosure. In order to have a RAID configuration you will need a minimum of 3 hard disks. For our purposes we will be using 5 hard drives in our configuration.
The computer must have a RAID controller inside the computer on the motherboard in order to connect the hard drives. The other option is to have an external RAID enclosure to hold the hard disks. With 5 disks in the configuration each block of data is being written and spread across those 5 drives writing a marker (parity bit) in case there is a loss of one of those drives. In the event of a drive failure the remaining 4 drives carry the load using the parity bit so it is not noticed by the end user of the computer. The parity bit keeps track of errors as each block of data is written to the hard drives. In the case of the loss of a second drive that would be catastrophic even for a RAID 5 configuration to handle. Usually RAID devices have LEDs on the drive enclosures that glow red in case of drive failure. The RAID unit manufacturer should provide alerting software that will work with your operating system and the system log to notify you of a failure as well. Once the failed drive is replaced the RAID unit will rebuild the data on the new drive re-establishing the RAID 5 set just like new. Recovery from the drive failure and the subsequent rebuild process may take several hours and will degrade the performance of your computer until completed.
Raid on motherboard is available from a few manufacturers. While not common in desktop PCs there are options allowing you to install external RAID enclosure with minimal effort.
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