The rednet API provides a simple computer networking model using modems.

Functions for specifiying the sides to be used for networking:
rednet.open( side )
rednet.close( side )
rednet.isOpen( side )

Functions for sending messages to other computers:
rednet.send( receiverID, message ) -- Send to a specific computer
rednet.broadcast( message ) -- Send to all computers

Functions for receiving messages:
rednet.receive( timeout ) -- Returns: senderID, message

Rednet is not the only way to use modems for networking. Interfacing with the modem directly using the peripheral API and listening for the "modem_message" event allows for more control.