To answer your first question, RAID is a method of connecting multiple HDs together to increase read speed, and/or to ensure multiple copies of data are created. There are various different types of RAID array (e.g. RAID 0, RAID 1 etc.), all with a specific purpose
Google the wiki article, it'll go into much better depth than I can.
Bear in mind though, that many studies have questioned the performance gain that can be obtained with RAID, and while some people go to great lengths to set up RAID 0 arrays for gaming computers, it is a heavily debated subject as to whether any useful performance gain is actually seen.
My view is that the gains are so small as not to be noticeable, with the opposite argument mainly sustained by those who have already spent large amounts on multiple hard disks, who don't want to think that they have wasted their investment.