Having 2 HD's the same and doubling the speed by splitting the data over the two.
If one fails you lose the lot as data is split over the two.
Having 2 HD's the same but the second is a mirror of the first.
So if the first fails then you willl have a backup on the second.
Im not sure on RAID 2 or 3 but I think you have to have 3 HD's for them.
Thats how I see RAID anyway but I may be slightly wrong which I am sure some kind soul on here will put right.