Hard drives commonly used in IBM compatibles fall into six major classifications. There are two distinct purposes for the technology; defining how to encode data upon the drive's media (the data encoding standard), and defining how to communicate with the host computer (the interface standard).
MFM is a data encoding standard. It is usually seen with 5.25" drives with the ST506/412 interface. MFM uses two cables, a 34-pin and a 20-pin. MFM was the original encoding/interface standard; it is an old standard and is not actively marketed.
RLL is a data encoding standard. It is usually seen with drives using the ST506/412 interface. RLL uses two cables, a 34-pin and a 20-pin. RLL puts 50% more data into the same space as MFM, increasing storage capacity and performance. Some MFM drives can withstand being RLL'd, but it's not without hazard. This is an old standard and is not actively marketed.
ESDI is an interface standard. ESDI drives are usually RLL encoded. ESDI uses two cables, a 34-pin and a 20-pin. For its time, it was generally seen as larger, faster, and more reliable. However, this is an old standard; it is not actively marketed.
Pronounced "Scuzzy", also known as SCSI-1, SCSI is an interface standard. Up to 7 devices can exist on 1 SCSI controller. SCSI uses one 50-pin cable. SCSI is fast and relatively intelligent, and is consequently seen in higher-end systems with diverse, higher-end peripherals (such as large drives, scanners, CD and DAT drives). SCSI usually requires an interface card, known as a host adapter.
SCSI is particularly useful in a multitasking environment, as it is concurrent - it can work on more than one task at a time. This feature generally puts it ahead of other competing technologies such as IDE and EIDE.
SCSI2 is an interface standard. It is an improved SCSI, and supports old SCSI devices. It supports more devices and comes in "fast" and "wide" (16-bit).
SCSI revised for the third time. This interface standard uses a 68 pin cable, and can have up to 15 devices attached. Older SCSI standards are supported.
IDE is an interface standard. IDE drives are usually RLL encoded. IDE uses one 40-pin cable. IDE drives have their controller onboard, rather than on a card. IDE is as fast as ESDI, and more intelligent.
EIDE is an interface standard. EIDE drives are usually RLL encoded. EIDE uses one 40-pin cable. Existing drive/BIOS translation techniques, specifically the Cylinder, Head, and Sector (CHS) translation technique, cannot support hard disk capacities greater than 528Mb. EIDE solves this limitation - and breaks the 528Mb barrier - via Logical Block Addressing (LBA) translation. LBA requires BIOS support, or support implemented by third party software (such as EZ-Drive). LBA supports up to 8.4Gb of storage capacity. EIDE supports standard IDE devices based on traditional CHS translation. EIDE usually supports fast Mode 3 PIO and Mode 4 transfers and multiword DMA transfers for faster data throughput. EIDE usually provides two IDE connectors, thus supporting four IDE or EIDE devices.
EIDE is also being used for CD-ROMs and some tape drives. EIDE is a good standard, and it is cheaper than SCSI, but it does not compare to SCSI.