Redundant Array of Independent Disks, or RAID, is a method of storing content on a number of hard drives at the same time. A RAID might be software or hardware depending on the HDDs which are used - physical or logical ones, still what’s common between them is that they all work as one single unit where information is saved. The main advantage of employing a RAID is redundancy since the info on all the drives is the same all of the time, so even in case one of the drives fails for some reason, the data will still be present on the remaining drives. The general performance is enhanced as well because the reading and writing processes could be split between different drives, so a single one won't be overloaded. There're different types of RAIDs where the effectiveness and fault tolerance can vary according to the particular setup - whether your data is written on all of the drives real-time or it's written on one drive and after that mirrored on another, what amount of drives are used for the RAID, and so on.