RAID is something all of us have heard about but very few of us understand, at least fully. So lets get off on the right foot. RAID stands for Redundant Array of Inexpensive (or Independent) Disks. There are a dozen or so theories as to why RAID was conceptualized, but the most accepted reason is that once upon a time, not long ago, disks were small and expensive. In order to provide a large amount of data you had to have a bunch of disks all mounted in a single file tree, which was a real mess. So, to solve this problem RAID was born. With RAID you could take a bunch of disks at create a big virtual disk out of them which made administration much easier and more logical. Over time RAID grew to include new solutions for old problems, like disk performance, redundancy, and scalability. And for any skeptics out there, tell me where I can get a 10 terabyte disk drive.... that should make us all agree that RAID has a place in the universe.
Just to try and clear things up a bit more, lets see why we don't simple just need RAID, but actually WANT it. Let's say we're building a production NFS server that will be used to store all of our software. We'll need this system to extremely stable, because if it goes down no one can get or submit code. With RAID we could build a single virtual disk (volume) that would meet our need for 200G of disk. But we also what to make sure that if disks die that we don't go down. So we use a mirror (another set of disks identical to the first set of disks). If a disk dies we're okey, because the mirror will take over; we essentially have 2 identical sets of the same data which are constantly kept up to date. See? Using these 2 simple RAID concepts we've achieved both availability (thats our mirror saving us from disk crashes) and increased capacity (we've got a whole bunch of disks working together, which is cheaper than buying a single 200G disks... if you can find one!).
Okey, enough of the bad examples. Lets look at the different forms of RAID in use today.