Parallel ports are a more recent invention and are much faster than serial ports.
USB ports are only a few years old, and will likely replace both serial and parallel ports completely over the next several years. That is, it takes a byte of data and transmits the 8 bits in the byte one at a time. The disadvantage is that it takes 8 times longer to transmit the data than it would if there were 8 wires. Serial ports lower cable costs and make cables smaller. Before each byte of data, a serial port sends a start bit, which is a single bit with a value of 0.