There are lots of different kinds of RAID. The wikipedia article is a good starting point. The main ones you'll be interested in are RAID 0 (striped) and RAID 1 (mirrored). Mirrored RAID is basically making a real-time backup of one drive onto another ie all writes are done to both disks at once. If one drive fails, you have all your data safe. Striped RAID improves read and write speeds by splitting the data across two disks. Basically it writes alternating blocks to each disk, treating them as one volume. In theory this doubles your throughput, although there are overheads. Problem is, if one disk goes down, your volume is toast, so you want ultra-reliable disks (Seagates) and regular backup. You can also do a combined 0+1 for speed and security if you have four disks. You can do RAID on any kind of disk, SCSI, ATA, SATA, Firewire, even flash drives and floppies. It's probably best to use a hardware RAID PCI card if you can afford one, as this will take load off the CPU. You can also get server motherboards with hardware RAID onboard. These will often also have two CPU sockets :D