Integrated Drive Electronics (IDE) hard disks have been around for quite a few years.  Prior to these drives, hard disks were interfaced to a PC motherboard via an expansion board known as a hard disk controller.  The drive did most of the mechanical stuff and performed basic electronic/servo functions; the controller told it in detail what to do.  The development of the IDE hard moved most of the electronics and firmware (low-level software on a chip) from the controller to a printed circuit board on the drive itself.  In the process, a buffer/cache' memory was added to the electronics to speed-up the process of reading and writing hard disk drive data.  The drive got "smarter."  Overall costs went down and performance went up.

To put it in a nutshell, the ATA Serial standard is a simplified packet switching network between a motherboard or computer backplane and a disk drive.  It employs balanced voltage (differential) amplifiers and four wires/two pairs (transmission line) to connect transmitters to receivers in a manner similar to the 100BASE-TX Ethernet. The pins in the spec are labeled TX+, TX-, RX+, and RX- just like they are in the twisted-pair Ethernet. There is no specification for a standard ATA Serial cable (just electrical requirements it must meet), but each pair of wires will probably be parallel and shielded (there is a cable construction example in the spec.).

Kits are widely available that convert most notebook 2 1/2" IDE hard disk drives from a 44-pin cable with power to a standard 40-pin IDE cable without power and a separate power connector.  Most of them include a mounting kit so the drive can be installed in a standard  3 1/2" drive bay in a desktop PC.