Hard disks have passed through a long evolution of connectivity and data transfer standards. The drive interface standard from the 1980's was called the IDE (Integrated Drive Electronics) specification. Today it's usually called PATA (Parallel ATA). IDE includes sub-standards with names like EIDE, ATAPI, ATA, ATA-1, ATA-2, UDMA, ATA-4, ATA-133, and more. These sub-standards represent continuing improvement in data transfer speed and reliability.
Throughout this evolution, IDE's physical connectors have remained the same. So you could take an old 386 drive from 1988 and plop it into your Pentium 4 from 2005, and it would work! The IDE standards applied across thousands of different drive and motherboard manufacturers.
Here's how the IDE connector sockets on the motherboard look, along with the ribbon cable that goes from the motherboard socket into the back of any IDE drive:
PATA Pictures
PATA Photos (Wikipedia and www.ComputerHope.com)
The third photo shows a close-up of an IDE drive power plug, the Molex 4-pin plug. It's white and it has a female connector that goes into the 4-pin male connector on the back of the disk drive.
The final photo shows the back of an IDE disk drive. You can see where the IDE data ribbon cable plugs in to connect it to the motherboard and where the Molex power plug goes. Note the jumpers. You can connect either one or two IDE drives to an IDE ribbon cable. You set the jumpers to indicate whether one or two drives on are the same IDE cable, and which drive is where in the sequence. (Connecting SATA disk drives is simpler because they eliminate jumpers.)