Serial communications

In telecommunications and computer science, serial communications refers to any data transmission scheme in which data is sent one symbol at at time, sequentially over a communications channel.

The communications links across which computers, or parts of computers, talk to one another, may be either serial or parallel. A parallel link transmits several streams of data (perhaps representing particular bits of a stream of bytes) along multiple channels (wires, printed circuit tracks, optical fibres, ...); a serial link transmits a single stream of data.

At first sight it would seem that a serial link must be inferior to a parallel one, because it can transmit less data on each clock tick. However, there are plenty of compensating advantages.

A serial connection takes up less space. That's good in itself, but it also means that ...
The extra space can be used to isolate it better from its surroundings.
Not having multiple conductors in close proximity means less crosstalk at higher frequencies.
Clock skew between the different channels is not an issue.
These last three considerations mean that a serial connection can, all else being equal, be clocked considerably faster than a parallel one.

Some examples of serial communication architectures:

Morse code telegraphy
RS-232 (old, low-cost, low-speed, for connecting computers to peripherals)
RS485
Universal Serial Bus (newer, moderate-speed, for connecting computers to peripherals)
FireWire
Fibre Channel (high-speed, for connecting computers to mass storage devices)
InfiniBand (very high speed, broadly comparable in scope to PCI)
Serial Attached SCSI
Serial ATA