Q:

What advantages do RAID installations provide?

"Redundant Array of Independent Disks "RAID" is a method of using multiple disks to provide different balances of increasing data reliability and/or increasing input/output performance, depending on the RAID level being used."
The main problem is I can't imagine what this would look like. Could someone explain with a practical example?

A:

RAID is simply put a way to make two or more disks act as one - with some benefits:
RAID 0 is making the disks act as one physical disk. this means that the controller can split the files over both disks when writing and reading, which can improve the performance quite a bit. This also means that if one of your disks fail, the data on both of them are likely lost.
RAID 1 is mirroring; you need two identical disks, and you only get the space of one (two 1TB disks in RAID 1 only gives a 1 TB disk). The benefit here is that this enables the system to read from two different disks at the same time, even though write times might suffer a bit. Another benefit might be that if one disk fails, the data is mirrored on the other disk, but it is not completely without risks (see wikipedia)
Higher raid levels are possible, but you should read about them on wikipedia.

A:

First of all RAID is not a substitute for a good backup strategy.
There are basically two concepts one uses distributed copies (RIAD 1 - also called mirroring, data is organized in blocks and every drive contains the same blocks) and another striping (RAID 0 - mostly done at block level with block level striping).
One advantage of mirroring mode (two or more drives holding the same data) would be, that you can easily get snapshots and backups from the system by just removing the drive from the system and archiving that drive. This seems like a poor mans backup and snapshot strategy since there are more sophisticated mechanisms available nowadays but that's still a nice example for using redundancy.
Another benefit of mirroring is that you can execute as many read operations as you have drives in the array at the same time. Linux software RAID has that capability, but for writing every drive has to write the same data.
With striping data is organized in stripes that cover two or more drives, so that workload can be distributed across multiple drives. The array is faster but offers no redundancy.
To have the benefit of both modes one can use combined modes such as RAID 10, which uses distributed copies of striped blocks. But this comes at the expense of using even more drives to store the same amount of data.
RAID 5 uses only striping but manages to keep redundancy by calculating parity data. It has different characteristics than RAID 10 and it's not always clear which mode serves a task better, sometimes it's RAID 5 and sometimes it's RAID 10.
Though on personal experience we had a failed RAID 5 array at work in a customers machine because somehow errors got into the parity data and the only way out was to set up a new array. Of course the colleagues ignored the advice from the qualifier at that time and just replaced one drive, but a month later the array was dead and lost.
Apart from Wikipedia I would recommend reading the Linux RAID Wiki as well as the manpages of mdadm and md.

