Serial Transmission: 
In Serial Transmission, data-bit flows from one computer to another computer in bi-direction. In this transmission, one bit flows at one clock pulse. In Serial Transmission, 8 bits are transferred at a time having a start and stop bit. 

Parallel Transmission: 
In Parallel Transmission, many bits are flow together simultaneously from one computer to another computer. Parallel Transmission is faster than serial transmission to transmit the bits. 

Serial Transmission are used for long distance & Parallel Transmission are used for short distance communication.