The primary survey is the third step in the Red Cross' Emergency Action Principles, taught in Standard First Aid and First Responder, it is also a step in most (if not all) emergency response approaches. The primary survey follows the scene survey and activating EMS if needed. The purpose is to assess the casualty for any life-threatening injuries or problems. Ergo, the primary survey should include checking level of consciousness, ABCs, and checking for deadly bleeding and signs of head or spinal trauma. The casualty should be treated in the order of the severity of his condition (i.e. if he's not breathing there is no need to check for spinal injury or bleeding), and once stabilized, the primary survey may continue and be followed by a secondary survey. The importance of the primary survey should be obvious, the goal being to ensure and maintain stable, healthy vital signs so that the responder may continue with the secondary survey and wait for EMS if required.
Red Cross, First Responder Manual: Second Edition.
Wilderness Medical Associates, Field Guide.