RS-232 stands for Recommend Standard number 232 and C is the latest revision of the standard. The serial ports on most computers use a subset of the RS-232C standard. The full RS-232C standard specifies a 25-pin "D" connector of which 22 pins are used. Most of these pins are not needed for normal PC communications, and indeed, most new PCs are equipped with male D type connectors having only 9 pins.
There is a bit of an issue with the term RS232. As Syed stated the current standard is RS232C requires +12 -12 volt line swings and won't register anything between +3 and -3 volts as a valid signal. The standard doesn't require all pins, it only requires a TX and RX line the rest are optional.
However almost no real world devices actually produce or require RS232C compliant signals. RS232 is more modernly used as a general term used when it's simple serial data, using a TX and RX line, often at simple 0 and 5 volt logic.
The section about Data Parity and Stop bits explains the actual logic levels of a typical serial byte stream, the most common convention I've run across is 8N1 which is 1 start bit 8 data bits, no parity bit and 1 stop bit. The start bit is required. Some systems use more than 1 start bit or some kind of more complicated frame systems but generally at that point it's not considered RS232 anymore.
Typical RS232 logic is inverted meaning a low logic state is actually a high voltage state and a low voltage state is a high logic state.
Many will not include this or know this. Teletypes once used 4-20mA current loops for communication to a modem. Bell System created an interface to connect modems (communication equipment) to terminals and computers (terminal equipment). The standard used voltage rather than current for data signaling. Was limited to (if I remember) less than 50 feet. And was only intended for modem connections. Handshaking signals also permitted modem and terminal equpment to know when a connection exists, data throttling, and even a ring indicator. Standard even made half duplex or full duplex communication possibe.
Most implementations of RS-232 were kludges. For example, DEC used a modified version, combined it with XON-XOFF to do data throttling, and connected to printers. From that kludge came a nine pin (kludge) version of the 25 pin RS-232.
An old joke: RS-232 is a "non-standard standard". And survived until a clearly superior standard -USB - from Intel and many friends finally replaced it.