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 de-9 serial port interface (or COM port on Windows) is based on, and is used to connect with and program some MicroControllers.
The actual standard describes more than merely the electrical characteristics, and even goes as far as some of the mechanical connector constraints.
The 9 pin connector is in theory, not properly RS232, but since many of the connections are no longer useful in modern computing, and motherboards that support this interface at all do not bother with those, it is a point which is merely academic.
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.