RAID is a technology that achieves more storage capacity at high speeds, redundancy and fault tolerance by combining multiple hard drives and treating them as a single logical storage unit. Fault tolerance implies the ability of a system to continue functioning as expected even when one or more hard drives fail or otherwise develop an error (fault). Data is copied across multiple hard drives and in the event of a disk failure data loss is avoided. Furthermore, with more hard drives combined the overall system benefits from increased efficiency and heightened performance.
RAID was originally developed for servers to create large storage at lower costs and quickly became popular for hobbyists and home users for many reasons including personal DIY cloud storage and at home media centers.