RS-232 is the electrical standard used on most serial ports. Note that this standard is a +/- 15 volt system and requires a FDMI converter to make it compatible with TTL logic levels.
First thing: the name RS-232 has been obsolete since eons. The "RS" means "Recommended Standard", but it's a well-established accepted standard now. Today it's known as EIA/TIA-232, but often just EIA-232 is used.
EIA-232 was originally designed, in pre-PC times, as the electrical interface between DTE (Data Terminal Equipment, the computer terminal) and DCE (Data Communication Equipment, the modem). With the advent of PCs it's been used more to connect computers directly together, without the DCE in between. To allow this, the straight DTE-DCE cable had to be modified to a so-called null-modem.
EIA-232 only specifies the type of connector and the pin assignments, and the voltage levels. The data format, UART is often used, is not part of the standard.
EIA-232 makes use of single-ended connections. The minimum for bidirectional communication is Signal ground, TxD (Transmit data) and RxD (Receive data).