In order to standardize serial communication between different devices the Electronic Industries Association (EIA) defined in 1960 the Recommended Standard 232 (or in short RS232).
RS232 is often referred to as serial interface although other serial interfaces do exist.
The standard of RS232 defines the timing and the voltage level used in a connection between a terminal and a modem.
Originally a 25pol DSub connector was suggested to be used in order to connect devices with each other.
However, soon 9pol DSub connectors where used as well.
RS232 does not define a protocol that shall be implemented but the way transmission shall technically be done.
In communication two data cables are used called TxD (transmission) and RxD (receive) in combination with a third line for ground (GND).
A logically 0 is represented by a voltage between +3 V and +15 V, while a logically 1 is represented by -3 V to -15 V.
The data transmission is done using a certain number of bits, which carry ASCII control codes with determined start and end of each transmission.