Simply stated, RS-232 allows computing devices to pass data to communications devices. Computing devices are normally classified as DTEs (Data Terminal Equipment), while communication devices are classified as DCEs (Data Communications Equipment).
For a DB-25 connector, the DTE-configured interface is male, while the DCE-configured interface is female. In order for RS-232 to work, DTE-configured connectors must always be plugged into DCE-configured connectors.