RAID 0: Disk Striping without parity requires a minimum of 2 hard drives and one controller. It offers the best read write speed, but at a cost of zero fault tolerance! If either the controller or a hard disk fails then you are at ground zero.

RAID 1: Disk Mirroring uses one controller and a minimum of 2 hard drives. Both of the hard drives need to be the same size, but if they are not then they need to be partitioned to the same size. An example would be if you had a 30GB drive and a 40 GB hard drive.
The mirror set would contain two hard drives at 30GB each. 30GB would be for data while the second drive would be a mirror image of the data. If a hard disk were to fail then a new drive could be installed and the mirror image would be taken from the good drive. If the controller fails you are out of luck.

RAID 1: Disk Duplexing uses 2 controllers and a minimum of 2 hard drives. Disk duplexing gives you just a little more tolerance than mirroring. You obtain the same usable disk space as with mirroring. But if a controller fails the second controller will continue to write data to the second drive, which will not result in any data loss, unless the second controller fails or the drive on the second controller fails!

RAID 5: Stripe Set with parity uses a minimum of 1 controller and 3 disks, with a maximum of 32 disks. An example would be if you were using 3 drives. Data is written in stripe sets across all three drives, and if a drive fails the data (parity) information from the other 2 good drives will rebuild the data on the newly replaced drive.