Redundant Array of Independent Disks, or RAID, is a way of saving content on a number of hard drives at the same time. A RAID can be software or hardware based on the HDDs which are used - physical or logical ones, however what is common between them is the fact that they all work as one single unit where your information is saved. The biggest advantage of using a RAID is redundancy because the information on all drives shall be exactly the same at all times, 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 will also improve since the reading and writing processes could be split between a number of drives, so a single one won't be overloaded. There are different types of RAIDs where the capabilities and fault tolerance can vary based on the particular setup - whether info is written on all of the drives real-time or it is written on one drive and after that mirrored on another, what amount of drives are used for the RAID, and many others.