Serial communication is the process of sending data a single bit at a time, or serially. Alternatively, in parallel communication, the data is sent in sets of simultaneous bits each on separate wires. Serial communication came about largely because of the cost of parallel cables, as each parallel bit would have to have its own wire. It's especially more cost efficient in the case where you are transferring data over long distances. There tends to be more complexity involved in parallel communication, and as factors, such as processor speeds have increased over the years, thus increasing the speed at which data can be serialized, serial communication has become much more widely used. Serial buses are now the primary way for devices to communicate with each other between different systems, as well as locally on the same circuit board.