Power, if you cant answer his question, then dont post. Raid comes in various forums, such as raid0 and raid1. Which are the most popular and widely used. Raid0 for example increase's data utilization, in simple terms your HDD can deliver data much quicker. Raid0/1 needs at least 2 HDD for this process to take effect. Raid0 uses 2 HDD as one. It divide's data in half, each HDD shares one half. Then when the time comes, each HDD loads each part of data togather. So in theory, you have 2 HDD working togather sharing work. Raid0 is very usefull. Now Raid1 needs 2 HDD as well. When raid1 is apply'ed you only get the storage of one of the excisting HDD. Because the second is a back up. Raid1 copys everything writing on the the prime drive, and copy's it to the second. So if that drive fail's, you have all your original data stored onto your back up.
There's also software RAID, and the above mentioned hardware RAID.There's also many more forms other than 0 or 1.
RAID=redundant array of inexpensive drives
Basically it allows you to group some drives together and make them either act as one, or provide "striping" or complete backup of the data so if a drive fails you dont lose anything.By making them work together in parallel like raid 0 you theoretically gain some speed in file transfers.By using other schemes you make it so your data is more secure and recoverable if a drive fails and theoretically faster file transfers.Raid 0 is becoming popular with gamers due to the increased file transfer speed(see above answers), but in the server world you'll all kinds of combos up raid 5.It isnt always done with hardware either, its easily done with software too.It can also be done with a variety of drives and cabling setups, teh advantage is you dont have to have identical drives with some of the setups, and its cheaper than using an expensive commercial solution.
There are ways around that though, for example in a large array with one different drive thats bigger you can partition it off to make it match the others and use that space for something else.You can even use a combo of scsi and IDE, lots and lots of posibilities there.A block device is a block device, doesnt matter what it is physically or even what filesystem it has on it, thats the beauty of RAID.