RS232 is a protocol based upon a Serial Data Stream. It is the standard that was once used to connect modems with computers, and is also used for other peripherals. It is the standard that the DB-25 and DB-9 serial port interface (or COM port on Windows) is based on, and is used to connect with and program some MicroControllers.
The standard describes the electrical characteristics, and some of the mechanical connector constraints.
A 9-Pin DB-9 connector used to be associated with the RS232 protocol and commonly available on computers until the early to mid 2000’s.
Because of the voltage differences between devices used in embedded systems, and those on board PC’s, a voltage converting chip is often used, the common example being the MAX232.