The original hard disk interface used in the PC world was developed in 1980 by Seagate
Technologies, to work with that company's 5 MB ST-506 hard disk. It was later revised to
support the 10 MB ST-412, which was the first hard disk drive model used in the IBM PC/XT.
You can obviously see where the name of the interface comes from, given this bit of
history! In common parlance, this older interface is sometimes called "MFM" or
"RLL", so you may hear someone refer to a drive using this interface as an
"RLL drive" for example. These actually refer to the encoding
method used for storing data on the disk. Both encoding methods were used on these
early drives. (In fact, RLL is used on some IDE and SCSI drives also, making it a poor
name for referring to drives of one particular interface.)
The ST-506/ST-412 interface differs from the IDE/ATA and SCSI standards in one very
important respect: the hard disks were "dumb", meaning there was no built in logic board as modern drives have. All of the smarts resided in
the controller card that plugged into the PC. This caused a host of problems relating to
compatibility, data integrity and speed, because the raw data from the read/write heads
was traveling over a cable between the controller and the drive. This interface also
required a lot more work on the part of the user, because while a newer drive ships with
an integrated controller card built into the drive that is optimized for that drive, these
older ones didn't have this, and therefore the person setting up the drive had to program
the interleave ratios and other factors into
the drive to achieve maximum performance.
By today's standards, this interface and the drives that use it are microscopic in
capacity (although enormous in physical size), slow, cumbersome, error-prone and
completely obsolete. You will never see ST-506/ST-412 used in a new system, and in fact,
it's hard to find them in any systems still being used, unless you look around or
know a very frugal person. 
You can recognize this interface in older systems by the
use of two ribbon cables (instead of the single cable used by IDE/ATA and SCSI). One of
the cables is 20 pins wide and carries data, and the other is 34 pins and carries control
signals.
Next: Enhanced Small Device Interface (ESDI)
