It is amazing how often this question comes up.
Here is a break down of your basic RAID arrays you will run into:
RAID 0: Takes a minimum of two drives, up to 4 (or more depending on your controller) and it strips the data into predetermined block sizes and alternates writing between the disks.
Advantage: higher throughput in larger file transfers, great for video editing, great for lots of random file access (think server esq situation, in general)
Disadvantage: One drive dies you lose everything, same seek times, has no affect on gaming performance, increases windows load time (you have to go through the RAID device boot sequences + windows boot sequence)
RAID 1: takes 2 drives and mirrors the data across the drives, meaning whatever it writes to one it writes to the other. Everything is duplicated, everything.
Advantage: Perfect real time data backup
Disadvantage: Wastes a full hard drive devoting it entirely to mirroring, no performance advantage, increases windows boot time, no benefit for gaming or video editing.
RAID 5: Takes a minimum of 3 drives. This is where you really need to get a separate RAID controller. The processor breaks up the data and writes the information to Drive 1, then copies it to drive 2, then the next set of information is written to drive 2 and a copy is written to drive 3, then the next bit of data is written to drive 3 and a copy is is written to drive 1, so on and so forth. The size of the array is total disks - 1. There is no dedicated backup drive in the array in the case of distributed parity (what I described). You can also have nondistributed parity, which is where there is a dedicated drive that the parity (backup information) data is written to.
Advantages: Throughput increase, though not as much as RAID 0, from what I remember. Provides more flexibility than RAID 1 while maintaining the fault tolerance of RAID 1. Great for large arrays, this is what you would typically find in a server environment (if not RAID 6).
Disadvantages: Takes alot of disks, no affect on gaming performance, increase windows load time, needs a dedicated processor for parity calculations for best performance, expensive
RAID 6: Same as RAID 5 except that the size of the array is total disks - 2 and parity information is written to two different drives. RAID 6 can tolerate 2 hard drive failures simultaneously and still not lose data.
RAID 10/01: Takes 4 disks, it is a combination of RAID 0 and 1. If it is RAID 10, the data is mirrored across two drives then stripped across the other two. If it is RAID 01, then it is stripped across two drives then mirrored across the other two drives
Notes: RAID 10/01 is generally too much effort, plus you lose two disks.
Bottom line: Most people say their RAID system is faster, which is true in select environments. You have to apply context. Hard drives are not the bottleneck in a gaming or multimedia playback context. If you are editing large video files, transferring large (and I mean gigabytes, if not terabytes) of information, or have to have MS word files open .0000001% faster, then RAID is for you. I have used RAID 1/0 and its not worth the effort.