RAID (Redundant Array if Inexpensive Drives) is a means of utilizing multiple hard drives for speed and/or mitigating hard drive failure. There are multiple levels of RAID:
Level 0 - Data striping across multiple disks. This RAID level provides a performance increase by "striping" data across multiple hard drives. It offers no protection from hardware failures.
Level 1 - Data Mirroring - this level doesn't provide any performance increase, but should you lose a hard drive, you won't lose your data. The con is you lose half the total disk space because of the mirroring, hence it is more expensive per GB.
Level 5 - Multiple disks with parity protection. This level offers a similar protection from hard drive failure to RAID 1, but doesn't cost you the hard drive overhead. i.e. 4 disks in a RAID 5, only one is used for parity protection, so you have the storage space of the remaining three disks. RAID 5 only "costs" you an additional hard drive, but the biggest drawback is the performance loss because of the type of protection.
Level 0+1 - combines the attributes of level 0 and level 1. Provides the performance increase with level 0, but the mirroring protection in level 1. Best alternative, but most expensive. Minimum # of indentical drives is 4.
A couple of things about RAID - it is meant to provide hardware performance increase and/or protection from hardware failure. You still should do backups of your data - RAID is designed at a hardware level, not at the data level. When using multiple drives in a RAID array, your chances of a hardware failure increase because you are using more than one drive. For OS and Apps (which don't change very often), using RAID 0 is benfitial for load/access times. You can use Norton Ghost to create an image of your partition and if you experience a failure, you have a backup image.
For your personal or gaming data, pictures, music, etc, if put into a RAID array, it should be either level 1, 5, or 0+1 for the reason that you don't want to lose your data because of a HDD failure. Regardless, you still want to back up this data in the event you have a virus or data corruption - something RAID cannot prevent from happening.
There are additional levels of RAID, but they are designed for enterprise use with specific disk usage strategies. Most home PC RAID controllers don't support other levels of RAID simply because they are not needed or benficial to a PC user.