Redundant Array of Independent Disks, or RAID, is a method of keeping content on multiple hard drives concurrently. A RAID might be software or hardware based on the HDDs that are used - physical or logical ones, still what is common between them is the fact that they all perform as one single unit where data is stored. The key advantage of employing a RAID is redundancy as the information on all the drives will be identical at all times, so even in the event that a drive fails for some reason, the data will still be present on the rest of the drives. The general performance is also enhanced because the reading and writing processes can be split between different drives, so a single one won't be overloaded. There're different kinds of RAIDs where the effectiveness and fault tolerance can vary according to the specific setup - whether information is written on all drives real-time or it's written on a single drive and afterwards mirrored on another, what number of drives are used for the RAID, and many others.