1.You could say it's a way to make two HD's act as one, a bit simplyfied but still...(I'll leave the rest to Opus :D

RAID....Redundant Array of Independent Discs.

A protocol where hard drives are either spanned or mirrored.

There are two major ways of using this.
One is striped. This is where multiple drives are told to act as one large drive. Here data is split amongst the drives to gain a little more extra performance from the system as more drives working together lets each drive handle a certain amount of data.

The second is Mirrored where one drive is duplicated to another. This is not always peformance oriented as the data needs to be copied onto another drive as it is being written to the main drive.

There is an option to do Striped and Mirrored. This gains you nothing in performance in any way! It takes away in the long run.

Now, if you do striped and one drive fails, well, you lose half of your data! That is why most large server systems have striped and mirrored.

Now there are terms for these modes. RAID 0, RAID 1 and RAID 0+1!

You do not need a motherboard that has RAID on it. In fact you truly want to avoid this! Sometimes this only bogs things down. You should always get a PCI RAID card as it will offer you more flexability. Giving you more options!

In the long run RAID is not absolutely necessary. It does have some advantages especially when doing extremely large track counts, possibly. It's really great when you are doing projects with scattered tracks. I.E...not all playing at once!

If doing just general home recording and so forth. Don't bother as the PCI bus in todays systems coupled with the 8MB Cached drives have the horespower to get you the performance you truly need!

Hope that Helps

Opus :D