RAID does mean Redundant Array of Independent Disks, which does already explain a little bit of the term. It allows you to combine several disks to an array so that they provide redundancy and/or more performance.
Generally there are several RAID levels (see Wikipedia as already mentioned). The more important levels are RAID 0, RAID 1, RAID 5 and combinations of the like RAID 10 which is a combined RAID of an level 1 and level 0 array.
If you want to use RAID for redundancy you cannot use RAID 0, but any other level. RAID 1 has the best redundancy, because all disks except one may fail, but you can only use the capacity of one disk. RAID 5 has a better ratio between redundancy and capacity, because you can use the capacity of all except one disks and any one disk may fail. Decide yourself which RAID level you want to use.
There are different ways how to implement RAID. You can have a RAID controller, which does the work for you. It's generally simply a PCI(e) card which has several (S)ATA ports and does RAID on hardware level (that's why it's called hardware RAID). You can configure the RAID in the controller BIOS setup. Some controllers also provide userspace tools which allow managing the RAID via Windows.
Another way how to implement RAID is via the operating system. Many OSs provide software RAID functionality. If you have connected more than one harddisk directly to the mainboard of your computer, the operating system may do the work for you.
The disadvantage of software RAIDs may be, that your CPU has to do the work for you, but generally the bottleneck is still I/O and not the CPU.
BUT: You should keep in mind, that RAID in no backup solution. It only provides redundancy by offering failsafe performance. If you accidentally delete some files or you lose them as a result of an filesystem error, your data also gets corrupted on the other disks of the array. It is still necessary to regularly backup your data!
To configure a software RAID using Microsoft Windows you can use the Disk Management of the Computer Management MMC (
compmgmt.msc). Windows XP only supports RAID 0 and RAID 1, RAID 5 is only supported by the server versions of Windows, but this guide may help you to enable RAID 5 support on Windows XP, if you want to.