Redundant Array of Independent Disks, or RAID, is a way of saving content on multiple hard drives simultaneously. A RAID could be software or hardware depending on the drives that are used - physical or logical ones, but what is common between them is the fact that they all operate as just one single unit where information is stored. The biggest advantage of using a RAID is redundancy as the information on all drives is exactly the same all of the time, so even if a drive fails for whatever reason, the data will still be available on the other drives. The general performance will also improve because the reading and writing processes could be split between a number of drives, so a single one can't be overloaded. There're different sorts of RAIDs where the functionality and fault tolerance may differ depending on the specific setup - whether your data is written on all of the drives in real time or it's written on a single drive and then mirrored on another, the number of drives are used for the RAID, and many others.