Serial VS Parallel - Legacy Technology Differences

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Parallel Transmission
Parallel transmission occurs across a parallel wire. Parallel wires are flat and think, constituting multiple, smaller cables. Each cable can carry a single bit of information (either representing a 1 or 0). A parallel cable can carry multiple bits at the same time, one for each cable. An eight-cable parallel wire, for example, could carry an entire byte of data. This results in faster data transmission per second, all things being equal.

Serial Transmission
Serial transmission occurs over a single cable, one bit at a time. This type of communication is named "serial" not simply because data travels one bit at a time, but also because these bits must be organized in a particular way so that transmissions can be organized and considered trustworthy. For example, a single transmission from a peripheral device using serial data might take only 6 bits, so the serial mechanism has a way to dictate how to signal things like an "end of transmission."

Advantages and DIsadvantages
Parallel connections are, all things being equal, faster due to a higher rate of transfer. However, parallel ports also require more hardware, making them more expensive to implement. Furthermore, data transfer rates have increased to such an extent that serial connections can transfer entire gigabytes per second. Serial connections are also easier to implement, making them the go-to hardware choice for plug-and-play peripheral devices such as external hard drives and MP3 players. -- http://science.opposingviews.com/difference-between-serial-parallel-data-transfer-1608.html