not sure if this is right in ralation to your cars ECU but generally CAN = Controler Area Network same as LAN = Local Area Network its just a means of transmiting data round the car.
OBD = On Board Diagnostics, its the software that works out if everything is within parameters and if not tries to correct it or warn you there is a problem.
This isnt a very good analagy but its like windows uses the LAN to talk to servers, printers and stuff where obd/obdII uses CAN to talk to other bits of the car like the steering which usually has its own ecu if its electric.
http://www.freescale.com/webapp/sps/...02WcbfNZnLNnms has stuff on CAN if your interested.
That make sense to you?