Most of the time, when you talk about HMOs, you're really talking about closed-panel HMOs -- the least expensive, but least flexible type of health plan. They also tend to be geared more toward members of group plans than individuals.

In exchange for a low co-payment (or sometimes no co-pay at all), low premiums and minimal paperwork, an HMO requires that you only see its doctors, and that you get a referral from your primary care physician before you see a specialist. If you can still pick up the phone, you'll probably need to get clearance before you can visit the emergency room.

An HMO may have central medical offices or clinics (such as those used by Kaiser Permanente), or it may consist of a network of individual practices. In general, you must see HMO-approved physicians or pay the entire cost of the visit yourself. HMOs have the best reputation for covering preventive care services and health improvement programs.