What is RAID?

Alright, I know I'm an idiot but I still don't understand the concept well. What exactly is it and what can it be used for the common user?

RAID: Redundant Array of Independent (or Inexpensive) Disks
- there is software raid and hardware raid, hardware raid is better.
- all raids basically take two or more hard drives and present them to the operating system as one drive.  you can configure this to enhance performance, reliability, or both (if you use a lot of drives)
this is what you can do with raid:
- raid can take two hds and interleave i/o operations on them, which increases performance since the os can do something with another drive while waiting for the first drive to finish.
- raid can automatically copy what you write on a drive to another drive, and rebuild the original drive in case of failure.  this requires 2 drives of like capacity and you only have totally capacity of 1 drive, but reliability is enhanced because it's an automatic backup.  read performace is not degraded but write performance is.
- with 4 or 5 drives you can combine these two functions.

someone should list the raid numbers.