RAID stands for "Redundant Array of Inexpensive Disks".
That means, if you have 4 10G drives and put them in RAID-5 you'd have one large 30G drive (yes, 30 not 40 - the last drive is used to store backup info). Access speed is higher, as data is spanned out on three drives. In case one of the drives should fail, you just take the faulty one out and plug a new one in, and RAID rebuilds itself. No data lost.
That's, of course, high-end RAID. That IDE RAID can most probaby do only RAID-0 and RAID-1, and can't do rebuilds in background.
Ok, now the RAID levels:
RAID-0 : mirror. 4 10G disks appear as one 10G disks, but youve got 4 copies of it.
RAID-1 : striping. 4 10G disks are stiped together and it appears like one 40G disk. No redundancy, if one disk fails you're b0rked.
RAID-5 : if you use 4 10G disks, one of them is used to store backup information and the rest are available to you. If one disk fails, RAID is able to rebuild itself.
RAID-10 : like RAID-0 + RAID-1. Two 20G RAID-1 stripes are in a mirror. Probably fastest.