Redundant Array of Independent Disks, or RAID, is a method of keeping content on multiple hard drives concurrently. A RAID can be software or hardware based on the HDDs that are used - physical or logical ones, still what is common between them is that they all work as one single unit where information is stored. The key advantage of employing a RAID is redundancy as the info on all the drives will be the same at all times, so even if one of the drives fails for whatever reason, the info will still be available on the other drives. The general performance is enhanced as well because the reading and writing processes can be split between a number of drives, so a single one will never be overloaded. There're different kinds of RAIDs where the effectiveness and fault tolerance may differ based on the specific setup - whether your data is written on all drives real-time or it's written on one drive and after that mirrored on another, what number of drives are used for the RAID, and many others.