Serial communication is used for all long-haul communication and most computer networks, where the cost of cable and synchronization difficulties make parallel communication impractical. At shorter distances, serial computer buses are becoming more common because of a tipping point where the disadvantages of parallel busses (clock skew, interconnect density) outweigh their advantage of simplicity (no need for serializer and deserializer (SERDES)). Improved technology to ensure signal integrity and to transmit and receive at a sufficiently high speed per lane have made serial links competitive. The migration from PCI to PCI-Express is an example.

Source: Wikipedia (http://en.wikipedia.org/wiki/Serial_communications)