In any English speaking first aid approach, ABC stands for Airway, Breathing, Circulation.
Airway: Is the airway open (patent)?
Breathing: Is the casualty breathing?
Circulation: Are there signs of circulation? Does the casualty have a pulse? Capillary refill? Cyanosis?
These three criteria form a hierarchy for treating an unconscious victim, i.e. an airway needs to be secured before a person can breathe and breathing implies circulation. Thus there is little sense in doing chest compressions on a breathing casualty since he must have a pulse. Similarly, a casualty with an obstructed airway will not benefit from CPR until a patent airway can be secured.
In an attempt to generalize the ABC mnemonic, the letter D is often added to represent "deadly bleeding" or "delicate spine". As well the letter C sometimes also represents C-spine (a cervical spinal injury).