Redundant Array of Independent Disks, or RAID, is a method of keeping content on several hard disk drives concurrently. A RAID might be software or hardware depending on the hard drives which are used - physical or logical ones, however what is common between them is that they all function as one single unit where your information is saved. The biggest advantage of using a RAID is redundancy because the information on all of the drives is exactly the same at all times, so even in case a drive fails for whatever 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 will be split between various drives, so a single one won't be overloaded. There're different kinds of RAIDs where the effectiveness and fault tolerance could differ depending on the exact setup - whether information is written on all of the drives in real time or it's written on a single drive and afterwards mirrored on another, the number of drives are used for the RAID, etcetera.