RAIDs

RAIDs are a collection of drives, that when put together, are faster and/or bigger and/or safer. A RAID without any special configuration other than a collective volume is called JBOD ("Just a Bunch of Drives").

RAIDs might have a cache drive that is faster than the rest. Think 4x spinning drives, plus a fifth NVMe.

RAID Configurations

RAID 0 - also called “striped”, this speeds your drives up considerably, but has zero redundancy. A regular 7200 drives has read/write (“R/W”) speeds of around 120 MB/s. A RAID 0 configuration of 4x 7200 drives has a 700 MB/s R/W. 4x2TB drives = 8TBs.
RAID 1 - also called “mirrored,” this keeps the drives mirrored for redundancy/back-up. The speed is just as fast as a single drive (so 120 MB/s @ 7200). Slower but safer. 4x2TB drives = 4TBs.
RAID 2, 3, 4 - I never use any of these so I couldn’t tell you what they are.
RAID 5 - this is a RAID configuration that gives you speed and parity. Best of both worlds. 4x 7200 drives has a R/W of ~450 MB/s. 4x2TB drives = 6TBs.