I used to work for a place that offered both types of plans the employees could choose from. For starters, the cost of each plan influences how they are utilized. The HMO plan was typically $1000-$2000 less expensive each year than the PPO plans.
HMO is a Health Maintenance Organization--In our part of the country (the midwest), it involved a network of contracted providers for which all health services were covered by the insurance plan. With this type of plan, a person chooses a PCP whom they must see (first), and if specialist care is necessary, the PCP serves the "gatekeeper" to those providers. The rules of this plan are typically very strict. The patient *MUST* use the providers in the HMO, or the patient must pay in full out of pocket.
PPO is a "Preferred Provider Organization" Again, there is a network of contracted providers for this type of plan. The plan typically covered all expenses (less a deductible, and visit co-pays) again, as long as you stay in network. If the patient chooses, they're allowed to see providers outside the network. However, the PPO will pay their non-contracted provider what the PPO considers "reasonable & customary" rates. The patient, then, is responsible for any charges above the R&C rates. Of course, the "dirty little secret" here is that what the PPO considers R&C is never what the doctor/provider considers R&C. This type of plan usually does not require PCP referrals to specialists. This type of plan typically costs more.
Both plans usually cover preventive care visits.
Is this what you meant?