This option is where you choose how much storage space you have on your computer. Only Western Digital drives are offered due to their quality products and their affordable prices.
The cheapest drive is IDE, which is an older technology now that SATA and SATA2 have come out, but it is still reliable. The SATA or SATA2 drives are faster and hold larger capacities.
RAID is where you can either have a complete and instant backup of everything that you do (just in case one of the drives fail), or allows you to tie two of the drives together to get much faster performance and double the storage capacity.
Please note that RAID configurations, as the prices state on the form, are only for configurations using the built-in RAID controller of the motherboard. A more capable RAID controller will cost extra.