HMO you need referrals to see doctors, you must stay within the HMO network, most plans just have a copay, and there is very rarely a deductable.
PPO you can go in or out of the PPO network, you may have a deductable and coinsurance when going out of network, very rare do you have deductable and coinsurance in network, mostly just copays, and you do not need referrals.
I work for Blue Cross Blue Shield, and I understand there is more too it, but this is just a general idea of each plan.
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