Redundant Arrays of Independant Disks has to major levels that the average user is concernced with. Level 0 (striping) and level 1(mirroring).
Striping is more about speed increases and mirroring is more about data protection. Basically you need to hard drives that are exactly the same plugged into a raid controller. If you have striping enabled, the computer will see the two seperate hard drives as one big hard drive and will write data onto both of them at the same time (half on one half on the other). This makes data access almost double in theory (though in practice it is less than this). The downside to this is that if one harddrive fails, you loss ALL your data.
Mirroring is when the computer writes exactly the same data on both harddrives, giving you a good data back-up. If one harddrive fails, the other one steps in and just keeps going.
Striping is probably the best one to go for, as the cost per megabyte of mirroring is very expensive. However make sure you back up your serious data if u are using striping.