Raid 0 - Uses two or more identical harddrives simultaniously and splits the data between both channels increasing data read and write speeds. However this raid mode is not recommended for important data because if one of your hard drives fail, there is no way to recover the data. This mode combines hard drive space from both drives into one larger virtual drive.
Raid 1: Uses two or more identical harddrives and sends the same data to both harddrives, (basically a live backup drive), doesn't increase writability but I've heard with tweaking it can speed up readability to Raid 0 speeds. If one drive fails your data is safe on your second harddrive. However your disk space is no larger than one of your drives by itself. This is very useful for those big important company documents and such, and a safe haven for those paranoids.
There are several possible raid configurations, I recommended searching google for those.
