A medical scheme (often called 'medical aid') is regulated by the Medical Schemes Act and must cover Prescribed Minimum Benefits at cost. Medical insurance, sold as a 'health insurance policy', is regulated under the Short-Term Insurance Act, doesn't have to cover PMBs, and typically pays a fixed daily or per-event rand amount rather than the actual cost of care.
SA example: A R650/month 'health insurance' paying R2,000/day in hospital is not medical aid. Go to a private hospital with a R150,000 bill and the policy pays a flat daily amount; you owe the rest.