Managed care plans differ mainly on two questions: must you stay inside the provider network, and do you need a primary care physician's referral (a "gatekeeper") to see a specialist?
HMO (health maintenance organization): generally covers care only from its own network, and in a gatekeeper model a specialist visit needs a primary care physician's referral. Non-emergency care outside the network is generally not covered, so the member pays the full cost.
PPO (preferred provider organization): has a network with the best benefit inside it, but still pays a reduced benefit outside it. PPOs usually do not require a gatekeeper referral.
POS (point-of-service): blends HMO and PPO. A member who stays with the primary care physician and network gets the best benefit, and at each point of service may choose out-of-network care for more cost sharing.
EPO (exclusive provider organization): uses a network like a PPO, but generally pays nothing outside the network except for emergencies.
Memory aid: HMO and EPO = network only (except emergencies). PPO and POS = you may leave the network, but you pay more.