Technical description: Receiving
WARNING: This page is about a different car, the 2001 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.
When a control module is transmitting, all the other control modules listen to it. They do this by looking at the difference in voltage between bus+ and bus-. (A voltage difference close to 0 V is interpreted as a one and a difference close to 5 V as a zero.)
What the various combinations of ones and zeros mean is determined in advance so that all control modules interpret the information identically. The message contains a description of what is sent and its value, e.g. "Coolant temperature 100°C" or "A/C On ".
Electrical interference, such as from a relay, will affect the voltage on both leads in the same direction. The difference in voltage will not be affected, however, and this makes the bus extremely insensitive to interference.