RS-232 cables were originally intended to link terminals to modems. The terminal is formally named a Data Terminal Equipment, abbreviated to DTE. The modem is formally named a Data Communications Equipment, abbreviated to DCE.
A standard RS-232 cable has a 25-pin D-type socket, which connects to the DTE, and a 25-pin D-type plug, which connects to the DCE. All 25 pins are connected, with pin 1 on the plug wired to pin 1 on the socket, pin 2 on the plug wired to pin 2 on the socket, and so on. The shielding of the cable is attached to the metallic cover on the socket.
RS-232 signaling is much more robust than the signalling of many other communications standards. Pins can be shorted, not connected or drive more than one output.
Signals are named from the point of view of the Data Terminal Equipment. So Transmit Data on the DTE is connected to Transmit Data on the DCE. The Transmit Data pin on the DTE actually transmits data, whereas Transmit Data pin on the DCE actually recieves data.