Multiple disks (MD) support in Linux is a software implementation of RAID (Redundant Array of Independent Disks). The basic idea of software RAID is to combine multiple inexpensive hard disks into an array of disks to obtain performance, capacity, and reliability that exceeds that of a single large disk.
Linux software RAID works on most block devices. A Linux RAID device can be composed of a mixture of IDE or SCSI devices. Furthermore, because a Linux RAID device is itself a block device, it can be a member of another Linux RAID device.
Whereas there are six standard types of RAID arrays (RAID-0 through RAID-5) in the hardware implementation, the Linux implementation of software RAID has RAID-0, RAID-1, RAID-4, and RAID-5 levels. In addition to these four levels, Linux also has support for another non-redundant array called "Linear Mode."
All levels of Linux software RAID are discussed in greater detail in the Software RAID HOWTO of The Linux Documentation Project (TLDP). One important thing to remember is RAID is not a substitute for backups.