RAID, the acronym variously explained as “redundant array of inexpensive disks” or “independent disks,” is critical on the A+, Network+, Security+ and Linux+ CompTIA exams (at the least).
Here’s some great LinuxMagazine articles that walk you through the variants:
“Introduction to RAID” at http://www.linux-mag.com/id/7924/
“Intro to Nested-RAID: RAID-01 and RAID-10” at http://www.linux-mag.com/id/7928
“Nested-RAID: RAID-5 and RAID-6 Based Configurations” at http://www.linux-mag.com/id/7931?hq_e=el&hq_m=1154685&hq_l=4&hq_v=55610542ec