The use of a PC and Sound Card is showing up in some old modes and a number of  some exciting new digital modes.  The inexpensive hardware and availability of a number of free programs, plus the ability to use the same setup for RTTY, PSK31, WFAX, CW, and others, is making this a popular setup.

The interface is very simple. 
Receiving
A simple connection from your speaker jack to the line input on your sound card is all you need.
Transmitting
The line out from your sound card is connected to your microphone input, usually through an inexpensive audio transformer, and a variable resistor to adjust the signal level.
A DSR or CTS line from a serial port on your PC is used to key the a relay or opto coupler to the PTT line in your micrphone connection.  VOX operation is also possible.
The sound card in your PC is used to decode and modulate the signals in your radio.