Redundant Array of Independent Disks, or RAID, is a way of storing content on a number of hard drives simultaneously. A RAID might be software or hardware based on the hard drives that are used - physical or logical ones, but what’s common between them is the fact that they all operate as a single unit where info is stored. The key advantage of employing a RAID is redundancy since the information on all the drives will be exactly the same all the time, so even in case one of the drives fails for some reason, the information will still be present on the rest of the drives. The general performance is enhanced as well since the reading and writing processes could be split between a number of drives, so a single one will never be overloaded. There're different kinds of RAIDs where the functionality and fault tolerance could differ depending on the exact setup - whether your data is written on all the drives in real time or it is written on a single drive and after that mirrored on another, what amount of drives are used for the RAID, and so on.