It is sometimes called a COM port or an RS-232 port, which is its technical name. If that’s not enough to confuse you, there are two types of serial ports — DB9 and DB25. DB9 is a 9-pin connection, and DB25 is, you guessed it, a 25-pin connection.
A serial port can only transmit one bit of data at a time, whereas a parallel port can transmit many bits at once.
Example of usage – “The serial port standard is RS-232. This standard is used for transmitting serial communication between devices, which are usually called data communications equipment (DCE) and data terminal equipment (DTE). The serial port uses a nine-pin (DE-9) connector or a 25-pin (DB-25) connector. Originally, the standard used 25 pins. Because many of the pins were unused and were quite bulky, the smaller DE-9 connector became popular.”