Critical Care is when a person seeks help for an acute injury or illness. An example is when someone comes in for chiropractic care after falling down stairs or when they are experiencing a headache. As the pain or problem subsides, the person stops care until the next problem or crisis arises.
Preventive care on the other hand, is when a person receives regular chiropractic check ups to correct any spinal misalignments before they start causing trouble in the body. In this approach, one doesn't wait to have a symptom such as back pain, headaches or feeling poorly to come in for help. Many times spinal misalignments (subluxations) are present and the person doesn't feel anything wrong initially. It can take weeks, months and sometimes even years for a person to have any symptoms.
Remember that an ounce of prevention is worth a pound of cure. Being consistent with your chiropractic care will give you much better overall health.