Ham Radio Software
      Control software - By connecting the computer to the radio via an RS-232 or USB port, the computer can control the transceiver as well as read the frequency for quick, accurate logging.

Depending on the protocol used by your radio, an interface is usually required to convert the computer's RS-232 voltage to the levels required by the transceiver.

    Sound Card Software - By connecting the audio output of your receiver to the microphone or line in input of your computer's sound card and the speaker output of the sound card to the microphone input of your transmitter, software running on your computer can send/receive RTTY, PSK, OLIVIA, Slow Scan TV, even CW.   This connection is ideally made through isolation circuits to preclude the possibility of ground loops.
Ham Radio Deluxe
Both of the following programs cover both of the above functions and are very well engineered systems (I've used both) and they are FREE!
It is important to realize that you can enjoy the sound card modes even if your radio does not have a computer interface.
Ham software can be divided into two groups:
Radio Mobile - This application uses high resolution altitude data to predict radio signal propagation.  It is particularly aimed at building effective VHF/UHF repeater systems.